Two-layer tensioned-web-over-slot die coating: Effect of die lip geometry

Tensioned-web-over-slot die coating (TWOSD) takes advantage of the elastohydrodynamic interaction between the curved web under tension and the coating liquid to sustain a very small coating gap that enables ultra thin coating at relatively high speed. When the product requires two liquid layers, dual slot TWOSD coating can be used to coat those layers simultaneously. In this case, the liquid pressure along the coating bead sets not only the web configuration and meniscus locations but also the interlayer separation point. An easy way to control the pressure distribution is through the die lip geometry.Here, we analyze the effect of four different die lip geometric parameters, e.g. the downstream lip radius, the mid lip radius, the downstream lip offset and the mid lip apex point, on the coating window of the dual slot TWOSD coating. Using the model proposed by Nam and Carvalho (2009c) and a direct tracking of flow features, the boundaries of the vortex-free operating window, area inside the parameter spaces that ensure a uniform coating without vortex inside the flow, were obtained and compared for each die lip configuration.We found that the mid lip radius is one of the important parameters to control the location of the upstream meniscus. Also the location of the interlayer separation point can be controlled by the lip offset and location of the apex point.     

Two-layer tensioned-web-over-slot die coating: Effect of operating conditions on coating window

Tensioned-web-over-slot die (TWOSD) coating deploys elastohydrodynamic interaction to control the distance between the moving substrate and the coating die lip surface in order to be able to coat an ultra-thin liquid layer. Dual slot TWOSD coating is designed to deposit two thin uniform liquid layers onto a moving web simultaneously. Like in the fixed-gap dual slot coating, the interlayer separation point needs to be at the downstream corner of the mid lip in order to prevent coating defects. Different flow features, like weeping, bead breakup and feed slot vortices, limit the range of operating parameters that ensures uniform coating, and define the operating window of the process. In this study, we analyze dual slot TWOSD coating flow by solving the Navier–Stokes equation coupled with thin cylindrical shell equation using the finite element method. The boundaries in the parameter space that define the operating window or vortex-free window are automatically computed by a direct tracking method of flow features. The effect of operating conditions, such as liquid viscosity, web tension and web speed, on the critical layer thickness at which the coating becomes non-uniform is determined by this study.     

The effect of shear‐thickening on the stability of slot‐die coating

Slot‐die coating is an economical roll‐to‐roll processing technique with potential to revolutionize the fabrication of nano‐patterned thin films at high throughput. In this study, the impact of shear‐thickening of the coating fluid on the stability of slot‐die coating was investigated. For the coating fluid, a model system fumed silica nanoparticles dispersed in polypropylene glycol was chosen. These dispersions exhibit shear and extensional thickening characterized through steady shear and capillary break‐up measurements. The critical web velocity for the onset of coating defect for different flow rates was measured, while the type of coating defect was visualized using a high speed camera. For the shear thickening particle dispersions, the coating failed through the onset of a ribbing instability. The critical web velocity for the onset of coating defect was found to decrease with increasing particle concentration and increasing fluid viscosity. The minimum wet thickness was studied as a function of capillary number for the particle dispersions and compared with a series of Newtonian fluids with similar viscosities. In all cases, shear‐thickening behavior was found to stabilize coating by reducing the minimum wet coating thickness when compared against a Newtonian fluid with similar viscosity at the same capillary number. Conversely, the shear‐thinning fluids tested destabilized the coating by increasing the minimum wet thickness when compared against a Newtonian at the same capillary number. The impact of shear‐thickening on slot‐die coating was further studied by quantifying the evolution of the ribbing instability with increasing web speed and by conducting tests over a wide range of coating gaps. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4536–4547, 2016     

A macroscopic mathematical model for tensioned‐web slot coating

A mathematical model was developed to predict the performance of tensioned‐web slot coating (TWSC), in particular, the minimum wet thickness. The model was based on the lubrication approximation for the coating solution flow and a simplified membrane theory for the moving web. The theoretical predictions compared reasonably well with available experimental data. The effects of fluid viscosity, web tension, coating speed, and wrapping angle on the performance of TWSC were evaluated. An example was presented to illustrate how die lip design could be varied to improve its performance. POLYM. ENG. SCI., 2008. © 2007 Society of Plastics Engineers     

Analysis of slot coating flow under tilted die

Slot coating is a high precision coating method, where the film thickness is controlled by the flow rate fed to the die and the production speed. The range of desirable operating conditions for uniform coating is limited by the shape and locations of upstream and downstream menisci, which are controlled by the pressure gradient within the coating flow. The gradient can be controlled by the shape and orientation of the slot coating die, that is, die configuration. Here, the tilted die, the so‐called angle‐of‐attack configuration is considered. The configuration is similar to underbite and overbite configurations, but it has a sloped die lip due to tilting. Coating flows with such a configuration are examined by computer‐aided analysis using the Galerkin/finite element method. Using steady‐state analysis, the effect of the angle of attack on the upstream meniscus location is discussed. In transient analysis, the amplitude of the thickness variation is predicted under different types of disturbances, namely flow rate and gap oscillations. The analysis shows that die lip configurations affect the thickness uniformity under periodic disturbances. The effect of die tilting can be similar to or different from the underbite/overbite configurations, depending on the type of oscillation. During the analysis, the flow rate apportioning inside the coating flow and decomposing thickness variations into two separating oscillations are useful in understanding the results is found. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1745–1758, 2015     

Frequency response analysis of slot coating

Frequency responses to the slot die coating process is analyzed using empirical modal analysis to predict the effects of periodic process disturbances such as gap oscillation and variations in vacuum pressure, web velocity, and flow rate. A type of empirical modal analysis known as an experimental modal approach was used, and an oscillator basis model was assumed by using a linearized governing equation, and the coefficient of the basis model was determined by curve‐fitting. By completing the process, we were able to decompose each mode, during which process it was found that the modes are of two types: a squeeze mode related to viscous characteristics and sinuous modes that are identical to capillary waves. Observation of the meniscus shapes of each mode revealed, in the third mode near the lip edge, significant fluctuations that can induce other coating defects. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Effect of sloped die lip geometry on the operability window in slot coating flows using viscocapillary and two-dimensional models

As an indicator for determining the operability window in slot coating flow, the viscocapillary model considering various configurations of upstream and downstream slot die lips was tested and compared with Navier–Stokes two-dimensional model. Bead pressure and sloped lip angle conditions for uniform coating operation demarcated from leaking and bead break-up defects were quantitatively predicted from the position of upstream meniscus from both models. By comparing the results, it is confirmed that the viscocapillary model for many kinds of sloped die lips could predict the operability window accurately. It is also found that there exists vortex or recirculation regimes inside upstream and downstream coating bead regions, depending on the angles of sloped die lips, even for the stable coating flow. The flow control by die lip structure will be usefully applied to design the strategy for the reliable and optimal coating process, including vortex-free windows.     

Operability coating windows and frequency response in slot coating flows from a viscocapillary model

Slot coating, indispensable to the manufacturer of flat panel displays and long-life secondary batteries, can be susceptible to unexpected disturbances at high speeds, leading to many kinds of undesirable defects. Operability coating windows for both Newtonian and non-Newtonian (shear-thinning) liquids have been investigated using a simplified viscocapillary model in a slot coating bead flow regime. Stable coating windows, free from leaking (or dripping) and bead break-up, have been determined by the position of upstream meniscus. They quantitatively coincided with those from two-dimensional calculations by a CFD Fluent solver. The pressure range that allowed a stable bead widened as the viscosity of the coating liquid or the capillary number in downstream die region increased. Also, the sensitivity of the slot coating flow through frequency response method was tested by measuring the amplitude of final wet coating thickness with respect to ongoing sinusoidal disturbances at different frequencies imposed to web speed, flow rate, bead pressure and coating gap. The viscocapillary model was compared with a 2D model and was found to be a fast and efficient tool that could enhance the productivity and processability of coating systems.     

Minimum wet thickness for double‐layer slide‐slot coating of poly(vinyl‐alcohol) solutions

A combined slide‐slot coating die, with the slide coating on top, was designed and built to investigate the double‐layer coating of poly(vinyl‐alcohol) solutions. The operating coating windows were examined as a function of flow rates and viscosities of the two coating layers. The top coating layer could be made much thinner as compared to the double‐layer coating so long as a stable thin film could be formed on the slide. A minimum wet thickness of the top layer was found to be as thin as 5 μm or less. A large viscosity ratio of the two layer solutions appears to be helpful in expanding the coating windows. Addition of a small quantity of polymer, such as carboxymethyl cellulose, can further enhance the coating speed and reduce the top layer thickness. A flow visualization technique was employed to observe the coating bead region. It was found to be easier to change the flow direction in the slide‐slot coating die than the double‐layer slot die, resulting in a more stable coating flow and much thinner top layer. POLYM. ENG. SCI., 45:1590–1599, 2005. © 2005 Society of Plastics Engineers.     

Effects of the molecular weight and concentration of poly(vinyl alcohol) on slot die coating

An experimental study was carried out to investigate the effects of the molecular weight and concentration of aqueous poly(vinyl alcohol) (PVA) solutions on the stable operating window of slot die coating. Various coating defects were observed outside the operating window. The window was found to expand with the PVA concentration and molecular weight increasing at low concentrations and to reach a maximum size at the gel point concentration, which corresponded to a critical Deborah number. Beyond this point, the effect of fluid elasticity became dominant, and the window began to shrink. This phenomenon was in contrast to that observed for low‐molecular‐weight glycerol solutions, in which the operating window was found to contract with the concentration or viscosity increasing. This anomalous behavior was attributed to the stretching or extension of long‐chain PVA molecules, which effectively stabilized the fluid motion in the coating bead region. The maximum coating speed could be correlated with the PVA concentration and molecular weight in terms of the capillary number as a function of the dimensionless concentration. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Transient response of two‐layer slot coating flows to periodic disturbances

Coating uniformity requirement is becoming more severe as new products come into the market. Coating processes have to be designed not only based on the steady‐state operation but also taking into account how the flow responds to ongoing disturbances on process conditions. These disturbances may lead to thickness variation on the deposited liquid layers that may be unacceptable for product performance. This study extends available transient analysis of single‐layer slot coating to determine the amplitude of the oscillation of each individual coated layer in two‐layer slot coating process in response to small periodic perturbation on different operating parameters. The predictions were obtained by solving the complete transient Navier–Stokes equations for free surface flows. The results show the most dangerous perturbations and how the deposited film thickness variations of each layer can be minimized by changing the geometry of the die lip and liquid viscosities. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1699–1707, 2015     

Coupled fluid‐particle modeling of a slot die coating system

This work seeks to develop a fundamental understanding of particle motion in the slot die coating process through studying the interaction of forces between particles, with the die walls and the fluid phase. Coupled computational fluid dynamics and the discrete element method is employed for evaluating the motion of individual suspended particles near moving surfaces in a complex three‐dimensional flow field, motivated by the flow of particle laden fluid in a slot die coating system, including the presence of free surfaces. Overall, the particles follow the flow streamlines and their final position in the coating depends on the initial entry region of the particles. Particles experiencing adhesion with each other agglomerate in the low velocity regions of the coating gap, and have long residence times near the edge of the die at the end of the feed slot in the coating gap. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1933–1939, 2016     

A review of the operating limits in slot die coating processes

Slot die coating is a pre‐metered process commonly used for producing thin and uniform films. It is an important film fabrication method for applications where precise coating is required. A major concern in slot die coating processes is how to determine the operating limits to set the appropriate range of operating parameters, including coating speed, flow rate, vacuum pressure, coating gap, liquid viscosity and surface tension, etc. Operating limits directly determine the effectiveness and efficiency of the process. In this article, the current state of academic research on operating limits in slot die coating processes is reviewed. Specifically, the theories, mechanisms, and empirical conclusions related to the limits on vacuum pressure, the low‐flow limit, the limit of wet thickness for zero‐vacuum‐pressure cases, the limit of dynamic wetting failure, and the limits of coating speed for a specific flow rate are reviewed. The article concludes with some recommendations for future work. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2508–2524, 2016     

Capillary pressure and viscous pressure drop set bounds on coating bead operability

In premetered slot or extrusion coating and related sheet coating a “bead” of liquid is held between the coating die and the moving sheet by capillary forces, which depend on gap clearances, surface tension, contact line attachment, and dynamic contact angle; by viscous forces, which depend on clearances, viscosity, meniscus, location, and coating thickness; and by the externally applied pressure difference, which must fall within bounds for the bead to be operable.New bounds are derived for quasi-static beads with variable meniscus location, extending Ruschak's [1] analysis. Viscous effects are modeled by Couette and Poiseuille contributions that account for nonuniform clearances and are important except in limiting cases like Ruschak's. Operating bounds are derived for viscous coating beads.     

Coating weight reduction by a carrier layer for tensioned‐web slot coating

The feasibility of using a dilute bottom carrier layer to reduce the thickness of a viscous top layer for tensioned‐web slot coating was examined experimentally. Aqueous poly(vinyl) alcohol solutions were prepared for both the carrier and the top layers. The solutions were coated on poly(ethylene terephthalate) films for observation. The thickness ratio of the two layers has to be in a certain range for stable coating, otherwise different types of coating defects would appear. The effects of viscosity, wrapping angle, surface tension, and web tension were studied. It was found that the viscosity ratio of the top layer to the carrier layer is the most critical factor; a large ratio is more helpful in reducing the thickness of the top layer. Increasing the wrapping angle, web tension, and surface tension of the top layer can also decrease the coating thickness of the top layer. The drying load is reduced substantially owing to the significant reduction of the top layer thickness. It was estimated that 50–90% of the energy required for drying could be saved by introducing a carrier layer in this study. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Experimental study on tensioned‐web slot coating

Tensioned‐web slot coating (TWSC) technology was developed in the early 1980s and was considered an efficient method for thin liquid film coating. An experimental study was carried out to investigate the effects of several key parameters on the minimum wet thickness of TWSC. The experiment was performed on a pilot coater with dilute Newtonian poly(vinyl) alcohol (PVA) solutions as test fluids coating on polyethylene‐terephthalate (PET) substrates. It was found that the minimum wet thickness for TWSC was between 0.5 and 1.5 μm, which is much lower than that obtained using conventional slot die coating. The minimum wet thickness was proportional to the tension number T_N, which is defined as the ratio of fluid viscous force over web tension. Furthermore, on the basis of the experimental data, two additional dimensionless groups: dimensionless pressure P_N and dimensionless surface tension L_N were found to be relevant to the performance of TWSC. A universal correlation involving these dimensionless groups as well as the die geometry was used to predict the minimum wet thickness of TWSC. POLYM. ENG. SCI., 47:841–851, 2007. © 2007 Society of Plastics Engineers     

Numerical simulation of the wire‐pinning process in PET film casting: Steady‐state results

Poly(ethylene terephthalate) (PET) film casting involves melt flow through a slot die, across a small span, and onto a fast moving quench drum. In the “wire‐pinning” casting process, a thin electrified wire close to the line of contact with the drum creates a strong pinning force that delays air entrainment to higher line speeds. Nonuniform wetting of the die lips by the extruded melt is thought to be responsible for the formation of streaks, a defect in the machine direction. A finite element model of the film casting process with wire pinning was developed to assist in understanding what causes significant wetting of the lips and whether this can be avoided by electrostatic pinning. The solution of the governing equations provide the location of the static and dynamic contact lines, thus finding the wetting and pinning points in the process. The simulations investigate the sensitivity of the static wetting line locations on the die lands to the imposed values of static contact angle and die‐lip gap. It was found that while the contact angle has a small effect on the extent of die‐lip wetting within the parameter ranges examined, there is a considerable chance that the feed slot surfaces can be dewetted. This seems to be the greatest danger for causing streaks rather than excessive wetting of the die lands. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Viscocapillary model of slide coating: Effect of operating parameters and range of validity

Slide coating is one of the premetered high‐precision coating methods. The layer thickness is set by the flow rate and web speed. The uniformity of the layer, however, can be affected by other operating conditions. Modeling the flow in the coating bead is necessary in developing the range of operability conditions where the layer is adequately uniform. Lubrication and viscocapillary models have been used to describe the flow and some of the operability limits of different coating processes. However, the available models of slide coating were developed with adhoc hypotheses that compromise their accuracy. We present a critical review of the available viscocapillary models and proposed changes to improve its range of applicability. The accuracy of the model is tested by comparing its predictions to the solution of the full two‐dimensional Navier‐Stokes equation. The model is valid at low capillary and Reynolds number regime and at low gap‐to‐wet thickness ratio. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Comparison of vertical and horizontal slot die coatings

Comparison of vertical and horizontal slot coatings was carried out experimentally using aqueous glycerin solutions with varying viscosity on a laboratory‐scale slot coater. The maximum coating speeds achieved in vertical slot coating were found to be higher than those of horizontal slot coating for low‐viscosity solutions. The coating defects observed below the minimum wet thickness changed from ribbing to air entrainment at a critical solution viscosity of around μ = 75 mPa s. Above this critical viscosity, a lower minimum wet thickness was obtained for the horizontal coating but not for vertical coating. The difference between vertical and horizontal coating was mainly the length of the coating bead. The dimensionless minimum wet thickness was correlated as a function of Reynolds number. There existed three regions of minimum wet thickness depending on Reynolds number. Initially, the dimensionless wet thickness increased in the low Reynolds number region (Region I), then reached a plateau in the intermediate Re region (Region II), finally dropped off sharply in the high Re region (Region III). The boundaries of these three regions were identified by two critical Reynolds numbers, i.e., Re = 1 and 20. Only Regions I and II of the wet thickness could be found for horizontal coating, while mainly Regions II and III were obtained for vertical coating, depending on the viscosity of the coating solution. POLYM. ENG. SCI., 47:1927–1936, 2007. © 2007 Society of Plastics Engineers