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     

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     

Analytical models for predicting penetration depth during slot die coating onto porous media

A series of analytical models have been developed to predict the penetration depth during slot die coating on porous media. Analytical models for both Newtonian and non‐Newtonian fluids were derived based on Lubrication Theory, Darcy's law, and a modified Blake–Kozeny equation. Using these models, the penetration depth can be quickly solved and the effects of material properties and processing conditions on penetration depth can be easily investigated. Experiments of coating Newtonian glycerin and non‐Newtonian blackstrap molasses onto Toray series carbon paper were conducted to validate developed models. The overall relative error between the predicted and measured penetration depth was found to be typically lower than 20%, which demonstrates the relative accuracy of developed models. Furthermore, based on a parametric study, it was found that the effect of capillary pressure on penetration depth is less than 10% when the ratio of coating bead pressure and capillary pressure is larger than 10. © 2014 American Institute of Chemical Engineers AIChE J 60: 4241–4252, 2014     

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     

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     

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.     

Optimization of air flow field of the melt blowing slot die via numerical simulation and genetic algorithm

The air flow field plays a key role in melt blowing. In this article, an optimal design procedure that improves the airflow field of melt blowing is proposed. A parameter, stagnation temperature which is a combination of static temperature and kinetic temperature, is proposed to evaluate the air flow field. The stagnation temperature is obtained via computer simulation, while optimization is accomplished by genetic algorithm. Four main geometry parameters of the slot die: slot width, nose piece width, slot angle, and setback are investigated. The optimal results were achieved in the 40th generation. The results also show that the smaller slot angle and larger slot width can result in the higher stagnation temperature. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

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 torque-based analysis of the reverse roll coating process

In a laboratory film coater operated at industrial speed, the torque exerted by the metering rod was used to investigate the hydrodynamics in the metering nip flow. CFD simulations were also performed taking into account the deformation of the roll cover to depict the elastohydrodynamics of the metering nip flow. From the experiments, it was found that the torque increases with transfer speed, load and viscosity. When runnability problems such as spitting were observed, the torque signal decreased. Torque measurements could also predict excessive friction between rod and roll, which may damage the elastic cover. The numerical simulations showed trends similar to the experiments. Regions of positive and negative torque values demonstrated how the fluid interacts with the rod, a phenomenon apparently related to the occurrence of secondary flows in the metering nip. Finally, the numerical results also showed that the minimum nip gap becomes constant at high loads.     

纺粘非织造窄狭缝牵伸器喷射流场的研究与分析

建立了纺粘窄狭缝牵伸器喷射流场的理论模型,采用有限差分法对该模型求解,采用SIMPLE算法求解速度和压力耦合,用交错网格解决速度和压力的锯齿状分布问题,差分格式为二阶迎风格式,使用交替方向的逐线TDMA方法求得差分方程。数值计算得到了气流速度在x方向上的分量,与实验结果吻合较好。通过对几种纺粘牵伸器喷嘴的喷射流场进行了数值模拟,给出了相应的流场矢量图,显示了该研究在对纺粘设备进行计算机辅助设计方面的应用前景。     

A coupled DEM/CFD analysis of the effect of air on powder flow during die filling

Die filling from a stationary shoe in a vacuum and in the presence of air was numerically analyzed using an Eulerian‐Lagrangian model, which employs a discrete element method (DEM) for the particles and computational fluid dynamics (CFD) for the air with a two‐way air‐particle interaction coupling term. Monodisperse and polydisperse powder systems have been simulated to explore the effect of the presence of air on the die filling process. For die filling with monodisperse powders, the influences of particle size and density on the flow behavior were explored. The numerical simulations revealed that the presence of air has a significant impact on the powder flow behavior, especially for systems with smaller and/or lighter particles. Flow has been characterized in terms of a dimensionless mass flow rate, and it has been shown that for die filling in a vacuum this is constant. The flow characteristics for die filling in air can be classified into two regimes. There is an air‐inert regime in which the particle size and density are sufficiently large that the effect of air flow becomes negligible, and the dimensionless mass flow rate is essentially identical to that obtained for die filling in a vacuum. There is also an air‐sensitive regime, for smaller particle sizes and lower particle densities, in which the dimensionless mass flow rate increases as the particle size and density increase. The effects of particle‐size distribution and adhesion on the flow behavior have also been investigated. It was found that, in a vacuum, the dimensionless mass flow rate for polydisperse systems is nearly identical to that for monodisperse systems. In the presence of air, a lower dimensionless mass flow rate is obtained for polydisperse systems compared to monodisperse systems, demonstrating that air effects become more significant. Furthermore, it has been shown that, as expected, the dimensionless mass flow rate decreases as the surface energy increases (i.e., for more cohesive powders). © 2008 American Institute of Chemical Engineers AIChE J, 2009     

Effect of doping in hydroxyapatite as coating material on biomedical implants by plasma spraying method: A review

Hydroxyapatite (HAp) is still one of the most widely used bioactive coating material to metallic implant in orthopedic fields because of its good biocompatibility, chemical and structural resemblance to natural bone, osteo-conductivity, coupled with quicker implant fixation and strong bonding between living bone with implants. Many techniques are used to deposit HAp as coating material on metallic implants among which plasma spray coating stands out as this process is cost effective, reliable, and protects surface of metal from wear and corrosion. Although, HAp is a smart choice as implant coating material, however, its medical application has been restricted because of the unfavorable mechanical properties like brittleness, weak fracture toughness and poor tensile strength. Further, HAp coated implants suffer from longer time period for remodeling, slow osseointegration rate and lack of antimicrobial effects/properties. Different methodologies have been adopted as surface modification techniques to increase mechanical as well as biological properties of HAp. Among those approaches use of dopants in HAp is a very efficient way for modification of properties. Therefore, aim of this review paper is to assemble information related to HAp coating by plasma spray technique on implants and discuss their advantages and limitations. The article also reports how addition of various doping ions into HAp can overcome these limitations by effecting structural, compositional, mechanical properties of HAp. Finally, it reports how the single, binary and multi ion dopants incorporation in the HAp structure can affect the properties which ultimately affect implant functionality when coated by plasma spraying method.     

A reactive-sputter-deposited TiSiN nanocomposite coating for the protection of metallic bipolar plates in proton exchange membrane fuel cells

To meet the needs of corrosion resistance and electrically conductivity for metallic bipolar plates that are employed in proton exchange membrane fuel cells (PEMFCs), a TiSiN nanocomposite coating was fabricated on to a Ti–6Al–4V substrate using reactive sputter-deposition through the double cathode glow discharge plasma technique. The microstructure of the TiSiN coating comprised nanocrystallite TiN grains embedded in an amorphous Si₃N₄ matrix. Electrochemical measurements were employed to investigate the corrosion behavior of the TiSiN coating in the simulated operating environments of a PEMFC, specifically 0.5 M H₂SO₄ solution containing different HF concentrations (namely 2, 4 and 6 ppm) at 70 °C pumped with H₂ at the anode and air at the cathode. With increasing HF concentration, a higher corrosion current density and lower corrosion potential were observed from both the coating and the uncoated substrate, indicating that the addition of HF accelerated their corrosion rates under these conditions. Compared to the uncoated substrate, the TiSiN coating showed a markedly higher corrosion resistance at all HF concentrations. The passive film that formed on the TiSiN coating, with a resistance of the order of magnitude of ~10⁷ Ω cm², displayed good electrochemical stability and was less affected by changes in HF concentration. For the TiSiN coating, the values of interfacial contact resistance (ICR) were 14.7 mΩ cm⁻² and 18.3 mΩ cm⁻², respectively, before and after 2.5 h potentiostatic polarization with 6 ppm HF under cathodic conditions under a compaction pressure of 140 N cm⁻². Both values are much lower than those for the bare substrate. Moreover, the TiSiN coating was shown to improve the hydrophobicity of Ti–6Al–4V that would help facilitate water management in the PEMFC operating environment. This coating, which exhibited excellent corrosion resistance, electro-conductivity and hydrophobicity, is therefore a promising material for protecting metallic bipolar plates from corrosive attack.     

New challenges in polymer foaming: A review of extrusion processes assisted by supercritical carbon dioxide

It is well known that supercritical carbon dioxide (sc-CO₂) is soluble in molten polymers and acts as a plasticizer. The dissolution of sc-CO₂ leads to a decrease in the viscosity of the liquid polymer, the melting point and the glass transition temperature. These properties have been used in several particle generation processes such as PGSS (particles from gas saturated solutions).It is therefore highly likely that extrusion processes would benefit from the use of sc-CO₂ since the rationale of the extrusion processes is to formulate, texture and shape molten polymers by forcing them through a die. Combining these two technologies, extrusion and supercritical fluids, could open up new applications in extrusion.The main advantage of introducing sc-CO₂ in the barrel of an extruder is its function as a plasticizer, which allows the processing of molecules which would otherwise be too fragile to withstand the mechanical stresses and the operating temperatures of a standard extrusion process. In addition, the dissolved CO₂ acts as a foaming agent during expansion through the die. It is therefore possible to control pore generation and growth by controlling the operating conditions.This review focuses on experimental work carried out using continuous extrusion. A continuous process is more economically favourable than batch foaming processes because it is easier to control, has a higher throughput and is very versatile in the properties and shapes of the products obtained.The coupling of extrusion and supercritical CO₂ technologies has already broadened the range of application of extrusion processes. The first applications were developed for the agro-food industry 20 years ago. However, most thermoplastics could potentially be submitted to sc-CO₂-assisted extrusion, opening new challenging opportunities, particularly in the field of pharmaceutical applications.This coupled technology is however still very new and further developments of both experimental and modelling studies will be necessary to gain better theoretical understanding and technical expertise prior to industrial use, especially in the pharmaceutical field.     

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.     

A novel set-up and a CFD approach to study the biofilm dynamics as a function of local flow conditions encountered in fresh-cut food processing equipments

The hydrodynamic conditions as well as design and surface properties within fresh-cut food processing equipment create a complex environment for biofilms. A new experimental approach was thus proposed to identify those physical parameters impacting biofilm development in such conditions. A set-up comprising original mock-ups mimicking generic features of washing tanks (e.g. welds, folds, flat surfaces, air/liquid/wall interface) was designed. The flow pattern therein was characterized using two computational fluid dynamic calculation approaches. Full trials were run for 48 h at 10 °C with a Pseudomonas fluorescens strain to identify the preferential biofilm formation areas. As in current industrial systems, the pilot rig had recirculation areas and low wall shear stress rates (τ_w < 0.1 Pa) in corners and angles. These were identified as critical areas with Surface Microbial Loads (SML) over 5 Log₁₀/cm². However, τ_w alone failed to explain why SML in areas under unidirectional flow was higher than in the mock-ups. Lastly, air/liquid/wall interface conditions were more critical than immersed surfaces. This study validated the possibility of using CFD methods to understand the way in which flow pattern influences biofilm formation. The methodology proposed would be helpful in quantifying equipment components criticality based on biofilm growth parameters.     

Fuzzy Traceability: A Process Simulation Derived Extension of the Traceability Concept in Continuous Food Processing

Liquid food production often involves continuous processing. This leads to problems in traceability systems due to mixing zones and therefore indistinct batch identities causing difficulties with regard to withdrawals or recalls. This article outlines the possible use of the concept of dynamic simulation to improve the handling of batch identities in continuous production of liquid food, a concept we call fuzzy traceability. The concept is illustrated with a realistic example from a real dairy process line.     

Realizing the enhancement of interfacial interaction in semicrystalline polymer/filler composites via interfacial crystallization

Polymer/filler composites have been widely used in various areas. One of the keys to achieve the high performance of these composites is good interfacial interaction between polymer matrix and filler. As a relatively new approach, the possibility to enhance polymer/filler interfacial interaction via crystallization of polymer on the surface of fillers, i.e., interfacial crystallization, is summarized and discussed in this paper. Interfacial crystallization has attracted tremendous interest in the past several decades, and some unique hybrid crystalline structures have been observed, including hybrid shish-kebab and hybrid shish-calabash structures in which the filler served as the shish and crystalline polymer as the kebab/calabash. Thus, the manipulation of the interfacial crystallization architecture offers a potential highly effective route to achieve strong polymer/filler interaction. This review is based on the latest development of interfacial crystallization in polymer/filler composites and will be organized as follows. The structural/morphological features of various interfacial crystallization fashions are described first. Subsequently, various influences on the final structure/morphology of hybrid crystallization and the nucleation and/or growth mechanisms of crystallization behaviors at polymer/filler interface are reviewed. Then recent studies on interfacial crystallization induced interfacial enhancement ascertained by different research methodologies are addressed, including a comparative analysis to highlight the positive role of interfacial crystallization on the resultant mechanical reinforcement. Finally, a conclusion, including future perspectives, is presented.