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     

Model for the outer cavity of a dual‐cavity die with parameters determined by two‐dimensional finite‐element analysis

A coating die forms liquid layers of uniform thickness for application to a substrate. In a dual‐cavity coating die an outer cavity and slot improves flow distribution from an inner cavity and slot. A model for axial flow in the outer cavity must consider the ever‐present cross flow. A 1‐D equation for the pressure gradient for a power‐law liquid is obtained as a small departure from a uniform flow distribution and no axial flow. The equation contains a shape factor dependent on cavity shape, Reynolds number, and power‐law index. The shape factor for five triangular cavity shapes is obtained by finite‐element analysis and correlated for application to die design up to the onset of flow recirculation which arises at the junction of the cavity and outer slot. The performance of the combined cavity and slot is considered and the most effective design determined. © 2017 American Institute of Chemical Engineers AIChE J, 64: 708–716, 2018     

Coating window for double layer extrusion slot coating of poly (vinyl‐alcohol) solutions

The expansion of coating windows for low‐viscosity poly(vinyl‐alcohol) (PVA) solutions is achieved via utilization of a higher viscosity PVA solution as the stabilizing carrier layer. It is found that PVA solutions of higher viscosities possess a higher degree of processability than those of lower viscosities, and can serve as a stabilizing carrier layer when coating lower viscosity PVA solutions. The stabilizing effect of the carrier layer enhances with increasing viscosity of the carrier layer, but reaches a saturation state at a high enough viscosity. It is further found that the coating defects occurring at lower maximum coating speeds are dominated by the lower, carrier layer, while those at higher maximum coating speeds are controlled by the upper, carried layer. Consequently, there exists an optimal layer flow rate ratio at which the coating window is the largest. A new type of coating defect, called spreading failure, is observed for coating situations involving a thin and much less viscous upper layer.     

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     

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     

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.     

Model for a two‐cavity coating die with pressure and temperature deformation

Coating dies uniformly distribute liquid for application as a film to moving substrates using one or two cavities spanning the coating width and adjoining precision narrow slots of much higher resistance to flow. If the slots are deformed by the pressure of the liquid or by temperature gradients in the die bars, degradation in flow distribution can result. Consequently, dies are designed to be sufficiently stiff and are thermally jacketed to keep slot deformations within fabrication tolerances. To aid in design and operation, a model of low computational load is developed in which the flow and deformation analyses are directly coupled. Die deformation is modeled using classical beam theory taking account of the varying thickness of the bars due to cavity geometry. Two‐dimensional finite element analysis of die deformation gives marginally higher slot deformation. Three‐dimensional finite element analysis agrees with the two‐dimensional analysis except near the center of the die where the symmetry boundary condition reduces deformation. The effects of die geometry on deformation and flow distribution are illustrated. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Start‐up of slot die coating

A flow visualization technique was employed to observe the start‐up of slot die coating. The effects of five variables, i.e., prewetting on the die surface, fluid viscosity, slot gap, coating gap, and die lip length on the start‐up time were investigated. Prewetting on the die surface can effectively reduce the start‐up time. Any variable that enlarges the steady‐state coating bead would increase the start‐up time. Therefore, increasing the fluid viscosity, coating gap, slot gap and decreasing the die lip length can be helpful in reducing the start‐up time. It was found that there exist four distinct modes for the establishment of steady‐state coating bead. The four distinct modes can be distinguished by a Reynolds number, which is defined as the ratio of the inertial force of the coating solution impinging on the moving web to the viscous force needed to pull the coating solution on the web. Comparison of the experimental observation with the theoretical prediction based on the commercial package Flow3D was also performed; the four modes can be found in the numerical simulations, the start‐up time computed to reach steady state is shorter. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

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.     

Linear stability analysis of two‐layer rectilinear flow in slot coating

Two‐layer coating occurs in many products. Ideally, the liquids are deposited onto the substrate simultaneously. In the case of two‐layer slot coating, the interlayer between the coating liquids is subjected to enormous shearing. This may lead to flow instabilities that ruin the product. It is important to map the regions of the parameter space at which the flow is unstable. Most of the stability analyses of two‐layer rectilinear flow consider the position of the interlayer as an independent parameter. Classical results cannot be applied directly in coating flows. We present a linear stability analysis of two‐layer rectilinear flow considering the flow rates as an independent parameter. The predicted neutral‐stability curves define the region of stable flow as a function of the operating parameters. The range of coating operating conditions is restricted further, when the condition for the desirable interlayer separation point location are considered together with the stability condition. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

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     

Process limits in two-layer reverse roll transfer

Reverse roll coating in which a thin single layer of liquid is applied onto a substrate has been used in industry for decades and has been extensively analyzed in the literature. Modern coatings, however, are often composed of more than one layer to improve the product performance and to reduce the manufacturing cost. Premetered methods such as slot, slide, and curtain coatings are typically used to produce such multilayer coatings. If the caliper of the substrate to be coated is not constant, then the coating gap and consequently the final film thickness deposited on the web will also be nonuniform. In this study, we focused on the use of reverse roll technique with slot die liquid delivery system to produce a uniform thin two-layer coating. The use of this coating technique to produce such a coating has not been previously explored. The liquid film surface as it is transferred from a rigid steel roll to a deformable urethane-covered roll was visualized to find out how the uniformity of the two-layer coating is affected by the speed ratio between two rolls, layers’ wet thicknesses, and liquid viscosities. The effect of these parameters on the ribbing frequency and amplitude was also investigated. The results show that in the two-layer coating, as in the single layer reverse transfer, there is a critical web speed above which ribbing occurs. The critical speed is determined by the bottom layer viscosity.     

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     

Response of slot coating flows to periodic disturbances

Slot coating is a common method in the manufacture of a wide variety of products. It belongs to a class of coating method known as premetered coating: in a steady-state operation, the thickness of the coated liquid layer is set by the flow rate fed to the die and the speed of the substrate moving past, and is independent of other process variables. Thus premetered methods are ideal for high precision coating. However, even the best designed slot coating operations are subjected to small oscillations on the process conditions, such as flow rate, vacuum pressure and gap fluctuations. These oscillation may lead to unacceptable variation on the thickness of the deposited liquid layer. The effect of process condition disturbances on the coated layer has to be minimized to assure a wet thickness as uniform as possible.The effect of an imposed periodic perturbation on the liquid flow rate or on the gap clearance in the coated layer thickness is explored in this work by computer-aided analysis. The amplitude of the thickness variation is determined at different process conditions and die configurations by solving the transient, two-dimensional, viscous free surface liquid flow in the coating bead. The system of equations, with appropriate boundary conditions, was solved by the Galerkin/finite element method, and an implicit time integration. The results show the response as a function of the imposed perturbation frequency and of the die geometry. They indicate that the die geometry may be optimized in order to minimize the film thickness oscillation of a slot coating operation.     

Optimization of operating conditions in a single‐zone drier for two‐layer polymer coatings

In industrial dryers, hot air is blown over wet coatings either from top or from both top and bottom sides to remove the solvent. Drying of multilayer coatings is a complicated process that involves simultaneous heat and mass transfer, transport, and thermodynamic behavior of polymer solutions. Airflow and its temperature are important operating conditions in a dryer. We report optimization of the operating conditions to minimize residual solvent without inducing blisters for two‐layer coatings using mathematical modeling. Our results reveal that optimal air flow on the bottom side of the coating is always greater than or equal to that on its top side—an agreement with the common notion of “backside drying.” Furthermore, the effect of coating thickness, residence time, individual layer thickness, initial solvent concentration, and reversal of the two layers on the optimal conditions is discussed. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

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.     

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     

Slot coating flows of non‐colloidal particle suspensions

Slot coating is used in the manufacturing of functional films, which rely on specific particle microstructure to achieve the desired performance. Final structure on the coated film is strongly dependent on the suspension flow during the deposition of the coating liquid and on the subsequent drying process. Fundamental understanding on how particles are distributed in the coated layer enables optimization of the process and quality of the produced films. The complex coating flow leads to shear‐induced particle migration and non‐uniform particle distribution. We study slot coating flow of non‐colloidal suspensions by solving the mass and momentum conservation equations coupled with a particle transport equation using the Galerkin/Finite element method. The results show that particle distribution in the coating bead and in the coated layer is non‐uniform and is strongly dependent on the imposed flow rate (wet thickness). © 2016 American Institute of Chemical Engineers AIChE J, 63: 1122–1131, 2017     

精密涂布工艺应用新进展

精密涂布工艺在功能性薄膜生产中起着重要作用。当今材料工业的迅速发展,对涂层提出更薄、更均匀的要求。特别象平板显示器(FPD)中所用的功能性光学薄膜,如防反射膜、防眩光膜等,其涂层厚度往往小于1μm。另外,锂电池电极的涂层则要求采用间歇式涂布方法来生产。本文着重介绍了近年来微凹版涂布和条缝涂布工艺在平板显示器、有机发光二极管(OLED)、锂电池行业中应用的新进展。