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     

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     

Substrate-induced coagulation (SIC) of nano-disperse carbon black in non-aqueous media: a method of manufacturing highly conductive cathode materials for Li-ion batteries by self-assembly

Substrate-induced coagulation (SIC) is a coating process based on self-assembly for coating different surfaces with fine particulate materials. The particles are dispersed in a suitable solvent and the stability of the dispersion is adjusted by additives. When a surface, pre-treated with a flocculant e.g. a polyelectrolyte, is dipped into the dispersion, it induces coagulation resulting in the deposition of the particles on the surface. A non-aqueous SIC process for carbon coating is presented, which can be performed in polar, aprotic solvents such as N-Methyl-2-pyrrolidinone (NMP). Polyvinylalcohol (PVA) is used to condition the surface of substrates such as mica, copper-foil, silicon-wafers and lithiumcobalt oxide powder, a cathode material used for Li-ion batteries. The subsequent SIC carbon coating produces uniform layers on the substrates and causes the conductivity of lithiumcobalt oxide to increase drastically, while retaining a high percentage of active battery material.     

Heat‐resistant hydrophobic–oleophobic coatings

Thermally and chemically durable hydrophobic oleophobic coatings, containing different ceramic particles such as SiO₂, SiC, Al₂O₃, which can be alternative instead of Teflon, have been developed and applied on the aluminum substrates by spin‐coating method. Polyimides, which are high‐thermal resistant heteroaromatic polymers, were synthesized, and fluor oligomers were added to these polymers to obtain hydrophobic–oleophobic properties. After coating, Al surface was subjected to Taber‐abrasion, adhesion, corrosion, and thermal tests. The effects of the particle size of ceramic powders, organic matrix, and heat on the coating material were investigated. Coating material was characterized by FTIR spectrophotometer. Surface properties and thermal resistance of the coating materials were investigated by SEM and TGA analyses. After thermal curing, contact angles of these coatings with H₂O and n‐hexadecane were measured. It was observed that coatings like ceramic particles are more resistant against scratch and abrasion than the other coatings. Also, they are harder than coatings, which do not include ceramic particles. It was seen that coatings, containing Fluorolink D10H, have high‐contact angles with water and n‐hexadecane. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2386–2392, 2006     

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     

Thermophoretic Coating with Molybdenum Oxide Nanoparticles

Thermophoretic and electrophoretic coatings are the main viable mechanisms for the coating of objects with nanoparticles. Unlike electrophoretic coating, thermophoretic coating has the advantage that electrically conductive substrates are not a requirement. This paper investigates the thermophoretic deposition and uniformity of molybdenum oxide nanoparticles, generated by a glowing wire generator, on various surfaces at three different flow rates (0.3, 1 and 1.5 L min^–1). The quantitative evidence of the presence of particles collected by a suggested thermophoretic precipitator at different flow rates has shown that a uniform distribution of the particles could be achieved across the whole area of the precipitator. SEM and TEM micrographs of the film confirmed that a homogeneous densely packed network of molybdenum oxide nanoparticles was built across the precipitation area at the flow rate of 1.5 L min^–1.     

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.     

Probing intermediates in the reaction of ethylene over supported Ru; use and limitations of multiple quantum spin counting

Multiple quantum spin counting is shown to yield an upper limit ofn = 6 for strongly bound, rigid intermediates C _m H _n in the reaction of ethylene over Ru/SiO₂. The results eliminate the possibility of major components of such intermediates being polymer-like coke precursers. A precurser to 2-butene is one possible intermediate.     

Effect of Cu strike coating on adhesion between Cu–Sn coated steel and rubber

In order to improve the adhesion between steel and rubber, a novel coating deposition technique has been developed, where steel substrate with orchestrated surface roughness was coated with double-layer coatings consisting of a thin Cu strike layer followed by a Cu–Sn layer with varying Sn compositions by immersion route. Coating surface characteristics studied using scanning electron microscope coupled with energy dispersion spectroscopy analyzer, electron probe micro analyzer, and inductively coupled plasma optical emission spectroscopy showed improvement in surface coverage with coating after employing the strike layer coating attributed to the better coating penetration in the deep roughness troughs. Peel test of the coated samples vulcanized with styrene butadiene rubber (SBR) was carried out which showed improvement in adhesion strength of the double-layer-coated samples inferring more uniform Cu-sulfide layer formation at interface due to more uniform coating coverage in these samples. Highest peel strength with uniform cohesive fracture within rubber was observed for optimum 2–3?wt% Sn content in the coatings. This result was further supported by pull-out test conducted on coated wire samples vulcanized with SBR.     

Bolaamphiphilic microstructural polyphenol flavonoids as sustainable high efficacy coating for aluminium surface in aqueous solution

Bolaamphiphilic polyphenol flavonoids were successfully revealed as a sustainable coating at the solid/liquid interface of an aluminium surface in an aqueous solution. Polyphenol flavonoids extracted from brown onions demonstrated the presence of bolaamphiphiles above 600 ppm. Characterization of the polyphenol flavonoids coating was performed using spectroscopic ¹H-nuclear magnetic resonance and attenuated total reflectance Fourier-transform infrared techniques for chemical identification, UV–vis and optical microscopy techniques were used for bolaamphiphilic microstructures assessment, and a scanning electron microscope was used for the assessment of surface morphology. Variant operating conditions used to show best coating efficacy were as follows: concentration = 600 ppm, solution pH = 10 in the presence of PO₄³⁻ ion cross-linker, operating temperature = 10°C, microwave pre-irradiation time = 5 s, and turbulent flow of the solution = 300 rpm. Maximum coating efficacy showed a coating efficacy of 97%. The suitability of several adsorption isotherms, like Langmuir, Temkin, and Freundlich, was tested to fit our data. Equilibrium constant values were in favour of successful coating, especially at lower temperatures (20°C). Spontaneous (negative ΔG°) and high affinities of coating material to the surface were revealed from thermodynamic parameters (ΔH° and ΔS°). Conclusively, such research is meant to emphasize our continuous support for the use of plant waste in artificial sectors such as coating of metals, and for their economic feasibility and low cost as high efficacy renewable materials.     

Particle Classification by the Tandem Differential Mobility Analyzer–Particle Mass Analyzer System

Particle mass analyzers, such as the aerosol particle mass analyzer (APM) and the Couette centrifugal particle mass analyzer (CPMA), are frequently combined with a differential mobility analyzer (DMA) to measure particle mass m_p and effective density ρ_eff distributions of particles with a specific electrical mobility diameter d_m. Combinations of these instruments, which are referred to as the DMA–APM or DMA–CPMA system, are also used to quantify the mass-mobility exponent D_m of non-spherical particles as well as to eliminate multiple charged particles. This study investigates the transfer functions of these setups, focusing especially on the DMA–APM system. The transfer function of the DMA–APM system was derived by multiplying the transfer functions of DMA and APM. The APM transfer function can be calculated using either the uniform or parabolic flow models. The uniform flow model provides an analytical function, while the parabolic flow model is more accurate. The resulting DMA–APM transfer functions were plotted on log(m_p)-log(d_p) space. A theoretical analysis of the DMA–APM transfer function demonstrated that the resolution of the setup is maintained when the rotation speed ω of APM is scanned to measure distribution. In addition, an equation was derived to numerically calculate the minimum values of the APM resolution parameter λ_c for eliminating multiple charged particles.

Copyright 2015 American Association for Aerosol Research     

Uniform Droplets Generation Using a Direct-Mode Jet Pulse Spray System

A novel method of preparation of uniform droplets using a direct-mode jet pulse spray system has been developed. By means of the periodical switch-on between a membrane-like rotating distribution exit and a micronozzle, a fluid stream was stimulated to be disintegrated into monodisperse droplets. The effects of the rotational speed (n) and thickness (H) of the distribution exit and flow rate of the dispersed phase (Q) on droplet characteristics were investigated. It was found that the uniformity of droplets would be enhanced with increased rotational speed and decreased thickness of the distribution exit, even for a relatively high flow rate of the dispersed phase. A relatively steady value of the coefficient of variation in size less than 10% could be obtained under the conditions of n = 350 rpm and H = 3 mm at Q = 7.5–8.7 mL/min. The influence of vortices was characterized by the Taylor number and parameter K, showing that the parameter K is capable of more precisely predicting the turbulence transition. Therefore, the system has effectively attained an improvement not only in uniformity but also in productivity of droplets generation.     

Inhibition of steel corrosion by potentiodynamic deposition of poly(N-methyl carbazole)

Poly(N-methyl carbazole) (PNMeCz) coating was deposited on 304 type stainless steel (SS) by electropolymerization of N-methyl carbazole monomer in tetrabutylammonium perchlorate containing acetonitrile solution using cyclic voltammetry. PNMeCz coating was characterized by attenuated total reflectance-Fourier transform infrared spectroscopy, scanning electron microscopy, thickness, conductivity, and contact angle measurements. Corrosion performance of the polymer-coated steel electrodes was investigated in 1 M H₂SO₄ solution using open circuit potential–time (E_ocp–t) curves, potentiodynamic polarization, and electrochemical impedance spectroscopy techniques. PNMeCz coating was found to provide anodic protection to the substrate and significantly reduce the corrosion rate of SS in acidic medium.     

The influence of power‐law rheology on flow distribution in coathanger manifolds

Coathanger dies are effective in delivering uniform flow if a polymer melt; however, when the fluid flow index varies from the design values, the flow is not uniform. Although mechanisms such as die lip adjustments have been effective tools for adjusting flow profiles, the issue of a variable flow index has not been fully addressed at the design stage. An analytical solution, based on the assumptions present in the 1‐D design equation, has been developed for the flow distribution in a coathanger manifold. This solution determines the flow distribution for a power‐law fluid with a flow index n* in a manifold designed for a separate flow index n*. From this solution, a uniformity index and a critical design angle are defined. The critical design angle is the angle at which the local derivative of the uniformity index with respect to n* approaches a maximum (for n* < n) or a minimum (for n* > n) as a function of the design angle. The critical design angle is independent of n and is presented as a function of the manifold aspect ratio.     

Automatic detection of contact lines in slot coating flows

Slot coating is a versatile method used to manufacture thin films at high speed. The success of the method lies in controlling a coating flow surrounded by upstream and downstream menisci. The meniscus edges that are in contact with either the die lips or the substrate surfaces are called contact lines. Visualizations of such lines are important in coating flow research because their shapes and locations are sensitive to operating conditions. In this study, we propose a robust image analysis algorithm for images acquired from flow visualizations. The images are dissected into three regions with different characteristics that need to be treated using different pre‐processing techniques. A standard optimal edge detector is then sufficient to capture the contact lines, and post‐processing steps can be simplified. We also highlight two applications of the proposed algorithm: coating windows, and transient behaviors under external disturbances. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2440–2450, 2017     

Mechanics of the low-flow limit in slot-die coating with no vacuum

Slot-die coating is a premetered, film-deposition process compatible with a wide range of materials. Of topical interest to precision electronics applications is the deposition of high-cost nanomaterial dispersions over moderately sized (>10 cm² ) areas with submicron wet film thickness. In this work, a two-dimensional (2D) model has been developed to understand the limits of the process and to predict the thinnest possible film achievable. Coined the low-flow limit, this parametric operating boundary presents the minimum uniform, defect-free film achievable at a given set of liquid properties and die/substrate geometry. We investigate the low-flow limit with a model that allows menisci to locate anywhere on the die lands, faces, and substrates with prescribed contact angles, thereby minimizing the assumptions on the bead configuration. The model is validated via comparison of its low-flow limit predictions to published experimental data. Analysis yields insights into the mechanics of coating bead breakdown at the low-flow limit.     

Operability limits of slide coating

Slide coating is one of the pre-metered methods used for high precision single and multilayer coatings. The thickness of each liquid layers is set by the flow rate and web speed only and it is independent of other process parameters. The uniformity of the deposited layer, however, is affected by the operating conditions. In the design of coating processes, it is crucial to know the set of conditions at which the deposited layer is adequately uniform, i.e. to define the operability window of the process. We developed a theoretical model of slide coating flow by solving the full two-dimensional Navier–Stokes equations and used it to uncover the mechanisms of coating bead breakdown at low vacuum, high vacuum, and low flow limits. With full understanding of the bead breakup processes, we then constructed a theoretical coating window as a function of coating thickness, web speed, and applied vacuum. A simple stability criterion was used to predict the onset of ribbing instability and deployed to add the onset of ribbing limit inside the coating window.     

Highly efficient separation of linear and branched C4 isomers with a tailor-made metal–organic framework

Separation of linear and branched isomers is of great commercial significance but still one of the energy-intensive and challenging processes in petrochemical industry. Here, we for the first time report a tailor-made metal–organic framework, ZU-36-Co (also termed GeFSIX-3-Co, GeFSIX = GeF₆²⁻, 3 = pyrazine), for the separation of both C4 linear/branched olefins (n−/iso-C₄H₈) and paraffin isomers (n−/iso-C₄H₁₀). With the pore size ranged between 3.82–5.25 Å, ZU-36-Co traps a large amount of n-C₄H₈ (2.35 mmol/g) and n-C₄H₁₀ (2.20 mmol/g) with iso-C₄H₈ and iso-C₄H₁₀ excluded. The molecular exclusion effect illustrates the significance of tailor-made pore window size for gas separation. To our knowledge, ZU-36-Co exhibits the highest n−/iso-C₄H₈ uptake ratio (13.8) and superior n−/iso-C₄H₁₀ separation performance compared with the state-of-the-art materials and sets a benchmark for the separation of linear and branched C4 isomers. This excellent molecular exclusion effect of ZU-36-Co was further confirmed by mixed gas breakthrough tests.     

Residence time distribution analysis from a continuous couette flow device around critical taylor number

This paper presents an experimental study of residence time distribution (RTD) analysis by pulse response technique in a continuous Couette flow device with rotating inner cylinder and stationary outer cylinder. Two kinds of experimental tests using pulses of tracer dye solution and particles resulting from a fast precipitation were performed in the region near the critical Taylor number characterizing boundary between laminar and laminar vortex flow. For most experiments performed in laminar and laminar vortex flow regime around the critical Taylor number over the ranges 0 < T_a < 120 and 0 < R_e < 5.5 the normalized response can be described by a dispersion model. The results of the critical Taylor number as characterized by the minimum dispersion number appear consistent with both theoretical predictions and other empirical observations.     

Flow visualization and operating limits of tensioned-web-over slot die coating process

Tensioned-web-over-slot die (TWOSD) coating is one of the most successful high-speed liquid coating process. It 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. However, flow instabilities that come from the gas–liquid interface and micro vortices inside the flow may lead to coating defects. Therefore, the range of operating conditions of uniform coating is limited.Nam and Carvalho [1] proposed a two-dimensional computational model to examine the role of the elastohydrodynamic interaction between the liquid and flexible substrate in tensioned-web-over-slot die (TWOSD) coating process, with the goal of predicting the operability limits of the process.Here, we use flow visualization on a laboratory-scale TWOSD coating apparatus to study limit flow states which are related to various flow instabilities and appearance of vortex in the flow. The visualizations show the progression of flow states beyond critical flow parameters which cannot be predicted by the model as three-dimensional features of these limit flow states. Furthermore, the critical flow rates, that define the operability window of the process, were determined experimentally and were used to validate the computational model.