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     

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     

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     

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     

Predictions of flow instabilities in the shear‐thickening regime with an improved bautista–manero–puig model

The trajectories of chaotic fluctuations in dilute micellar solutions, which typically occur during the creation and destruction of shear‐induced structures in the flow inception of a 0.4 wt % cetyltrimethylammonium tosilate aqueous solution and an aqueous solution of 3 mM cetyyltrimethylammonium bromide‐sodium salicylate, are predicted here with an improved Bautista–Manero–Puig model. To determine the constancy of the model parameters with time, we applied successfully an integro‐differential approach. This methodology revealed that all the model parameters, but the elastic modulus is constant. The nature of the predicted fluctuations was analyzed by employing the exponent of Lyapunov. This analysis demonstrates that the fluctuations are chaotic deterministic in Regimes I and IV, whereas they are random in Region II. We found that the predictions of the modified model follow the experimental data and reveal that these fluctuations are related to the creation of elastic structures at the inception of shear flow. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3735–3745, 2018     

Three‐dimensional numerical simulation of coalescence and interactions of multiple horizontal bubbles rising in shear‐thinning fluids

The dynamics of multiple horizontal bubbles rising from different orifice arrangements in shear‐thinning fluids was simulated numerically by three‐dimensional Volume of Fluid method. The effects of bubble size, rheological properties of shear‐thinning fluids, and orifice structure arrangements on multiple bubbles interaction and coalescence were analyzed, and the mechanisms of bubble coalescence and breakup were fully discussed and elucidated. The variation of bubble rising velocity during coalescence process and freely rising processes for different orifice arrangements was also deeply investigated. The critical initial horizontal intervals for coalescence of multiple horizontal bubbles with various orifice arrangements were attained by simulation, which could serve as the critical criterion of bubble coalescence or noncoalescence. Furthermore, the critical bubble interval was predicted based on the film drainage model, the prediction accords well with the simulation result and is quite conducive for the design and optimization of perforated gas–liquid contact equipment. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3528–3546, 2015     

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     

Simulation of particle migration in free‐surface flows

The migration of particles in free surface flows using the diffusive flux model was investigated. As the free‐surface flows, a planar jet flow and a slot coating flow were chosen. The suspension was assumed to be a Newtonian fluid with a particle concentration dependent viscosity. The governing equations were solved numerically by the finite volume method, and the free‐surface problem was handled by the volume of the fraction model. The result shows that even though the velocity profile is fully developed and becomes flat, the particle distribution never reaches the uniform distribution for both of the cases. It is also shown that the die swell of the free jet is reduced compared to the Newtonian fluid and when the particle loading is 0.5, die contraction is observed. The change in die swell characteristics is purely due to particle migration since the suspension model does not show normal stress differences. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Assessment of length and bundle distribution of dilute single‐walled carbon nanotubes by viscosity measurements

Rheological measurements have long been an invaluable technique in studying mechanical and structural properties of polymers. Measurements on dilute, noninteracting polymer solutions allow the determination of macromolecular structural information, such as molecular weight. This analysis has been complicated by molecular polydispersity; thus, average effects are more commonly reported. Here, we demonstrate polydispersity characterization for rod‐like polymers like single‐walled carbon nanotubes (SWCNTs). By extending the theory of the rheological behavior of rigid rods, we determine the distribution of length and bundle size in suspensions of SWCNTs by a simple rheological method. The method is based on measuring the viscosity of dilute suspended SWCNTs over a shear rate range spanning the Newtonian and shear‐thinning regimes. We show that a log‐normal distribution in length with minimal bundling accurately describes the viscosity measurements. This rapid new method yields the SWCNT length distribution while relying on bulk samples, which are less prone to artifacts. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1499–1508, 2014     

A Unified Model for the Mixing of Non‐Newtonian Fluids in the Laminar,Transition, and Turbulent Region

Summary: Non‐Newtonian fluid behavior has significant influence on quantities in chemical engineering like power input, mixing time, heat transfer etc. In the laminar flow region, the concept of effective viscosity by Metzner and Otto is well established. In the transition region between laminar and turbulent flow, the existing concepts use three and even more empirical parameters to determine the specific power input. Here, a unified and general but simple approach is introduced to calculate the power input for shear thinning fluids over the whole flow region using just one empirical parameter. The Metzner‐Otto relation is obtained as a limiting case for the laminar region. The empirical parameter of the new approach is related to the Metzner‐Otto constant. The concept is validated for eight different stirrer systems. Mixing time and maximum shear rate and heat transfer can also be calculated using this approach. The new concept presented should also be applicable for other apparatuses, e.g., static mixers.

Comparison of experimental data and a curve calculated according to the new method (solid line).     

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。     

钠基蒙脱土在涂料中增稠防沉降效果的研究

研究了钠基蒙脱土在涂料中的增稠及防沉降效果。将钠基蒙脱土配制成蒙脱土凝胶,采用前期加入和后期加入两种方法用于乳胶漆涂料中,对比分析对涂料的增稠和防沉降效果的影响。研究结果表明,钠基蒙脱土在乳胶漆涂料中可以代替部分有机增稠和防沉降剂使用。     

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     

Thermal plasma production of fumed silica: The effect of quench conditions on thickening and thixotropic ability

The effect of quench conditions on the thickening and thixotropic ability of fumed silica was studied. The powder was generated by rapidly quenching a vapour stream containing SiO_(g) with steam. Silicon monoxide vapour was produced by decomposing quartz in a transferred arc reactor using an Ar/NH,₃ thermal plasma. The parameters investigated included pre-quench temperature (1100-2100 K), and supersaturation ratio (1 - 10⁷, quench rate (17000 -75000 K/s), and quench stoichiometry (27 - 84). which was defined as the molar ratio of steam to SiO. The thickening and thixotropic ability of the powders increased with increasing quench rate and decreasing pre-quench supersaturation. This was due to an increase in specific surface area. Use of large H₂ O/SiO ratios caused a high degree of surface hydroxylation.     

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     

Effect of Zr content on friction and wear behavior of Cr‐Zr‐N coating system

Nanostructured Cr‐Zr‐N thin film with different Zr content (0 to 48.8 at.%) was deposited, using an RF magnetron‐sputtering technique. The structural evolution and morphological changes were performed. The tribological performances were evaluated, using a ball‐on‐disk type Oscillating tribometer. The tests were carried out under normal loads of 2, 4 and 6 N, respectively, with an alumina ball (Al₂O₃) as a counter face. The results showed that the crystallite size of the Cr‐Zr‐N system was reduced to 10.8 nm at 31.8 at.% Zr content. Morphological studies of the films showed that the roughness continuously decreased with increasing Zr content, exhibiting a value of 11.2 nm at 31.8 at.% Zr. The wear rate tends to decrease with the increasing of Zr content to reach a lowest value of 1.95 × 10‐2 μm3.N.μm‐1 at 31.8 at.% Zr. The wear rate and friction coefficient were lower in the samples with 31.8 at.% Zr content. The improved friction and wear resistance were attributed to the grain refinement strengthening mechanism at 31.8 at.% of Zr.     

Relationship between the structure and mechanical properties of polypropylene: Effects of the molecular weight and shear‐induced structure

A series of homopolymer polypropylenes (PPs), within a weight‐average molecular weight (M_w) range of 100–1600 kg/mol, were manufactured as dumbbell microspecimens. The effects of the molecular weight and shear‐induced crystallization on the mechanical properties and morphology were studied to gain a better understanding of the structure–property relationship. The results showed that the crystallinity decreased from 50 to 41% and the lamellar thickness increased as M_w increased. Tensile tests demonstrated that the stiffness and especially the tensile strength rose to extremely high values (Young's modulus = 2400 N/mm², stress at 30% strain = 120 N/mm²). Furthermore, the strain hardening effect was strongly affected by the lamellar thickness and highly oriented superstructures. Dynamic mechanical analysis demonstrated that the mobility of the molecular chains depended on M_w and on the lamellar thickness. In addition, the viscoelastic properties of unannealed and annealed samples indicated further the existence of shish‐kebab structures caused by shear‐induced crystallization during injection molding. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 519–533, 2007     

A study on the effect of presence of CeO2 and benzotriazole on activation of self‐healing mechanism in ZrO2 ceramic‐based coating

In this study, the effect of presence of CeO₂ and benzotriazole inhibitor agent on activation of self‐healing reactions and the corrosion behavior of ZrO₂ ceramic‐based coating are evaluated. The ZrO₂ and ZrO₂‐CeO₂‐benzotriazole ceramic‐based coatings were synthesized using sol–gel process and heat treated at 150°C. Afterward, X‐ray diffraction analysis (XRD), and Field Emission Scanning Electron Microscopy (FE‐SEM) were utilized to evaluate the phase analysis and morphology of these coatings. In addition, Energy Dispersive Spectroscopy (EDS) was used for elemental analysis of obtained coatings. Corrosion and self‐healing behavior of the coatings were investigated in 3.5 wt% NaCl solution using Electrochemical Impedance Spectroscopy (EIS) and potentiodynamic polarization tests. The results of XRD analysis revealed the amorphous nature of both coatings. FE‐SEM observations and EDS analysis results showed the presence of benzotriazole inhibitor agent and self‐healing reactions in the cracks and defects of ZrO₂‐CeO₂‐benzotriazole ceramic‐based coating. Moreover, results of electrochemical tests revealed that the presence of CeO₂ and benzotriazole inhibitor agent in the ZrO₂ ceramic‐based coating results in intense increases in the corrosion resistance of this coating by activating the self‐healing mechanism and forming passive layers.     

The effect of substrate roughness on air entrainment in dip coating

Dynamic wetting failure was observed in the simple dip coating flow with a series of substrates, which had a rough side and a comparatively smoother side. When we compared the air entrainment speeds on both sides, we found a switch in behaviour at a critical viscosity. At viscosity lower than a critical value, the rough side entrained air at lower speeds than the smooth side. Above the critical viscosity the reverse was observed, the smooth side entraining air at lower speed than the rough side. Only substrates with significant roughness showed this behaviour. Below a critical roughness, the rough side always entrained air at lower speeds than the smooth side. These results have both fundamental and practical merits. They support the hydrodynamic theory of dynamic wetting failure and imply that one can coat viscous fluids at higher speeds than normal by roughening substrates. A mechanism and a model are presented to explain dynamic wetting failure on rough surfaces.     

The effect of shear on the crystallization of cocoa butter

This paper examines the effect of shear on the crystallization of cocoa butter using a combination of three different experimental techniques and a single crystallization temperature of 20°C. Rheological measurements were carried out to study the effect of a shear step on the crystallization kinetics of the fat. Without a shear step, little rheological change was observed at 20°C; however, with the application of a shear step the onset of significant rheological change occurred and was strongly influenced by the magnitude of the shear step. Detailed crystallographic measurements could be made with in situ X-ray experiments during flow-induced crystallization. The imposition of continuous shear changed both crystal polymorphic structure and crystallization kinetics in a systematic way. Finally, optical measurements were used to follow changes in crystal morphology as a consequence of continuous shear. These results revealed the form and kinetics of crystal growth. In general the results complemented each other, and an overall picture of the way shear influenced cocoa butter growth could be formed. The observations could be the basis for a future mathematical model of growth kinetics and provide insight into the way shear influences crystallization kinetics, morphology, and polymorphic structure.