Visualization study of liquid surface stability for full reverse 3-roll coater with rigid gravure roll

Roll coating systems are widely used to coat a thin liquid layer on a moving substrate, and many publications discussing forward roll coating systems and reverse gap control coating systems are available. Recently, thinner and more uniform layers are required for use with the high-performance solutions involved. Moreover, high line speed coating is also required in order to reduce manufacturing costs. A reverse gravure roll system can coat a thin layer film at high line speed more easily than a flat roll coating system because only the liquid that enters the gravure cell is transferred to the next roll or substrate. However, the existence of the gravure cell complicates the flow between the rolls. To our knowledge, no systematic study in the literature has explored this condition in depth. In the present study, the flow between a reverse deformable roll and a solid stainless steel gravure roll is visualized in order to determine how the uniformity of coating in the high roll speed region is affected by operating parameters, that is, the speed ratio between the rolls and the properties of the coating liquid. The range for coating uniformity is compared with the case of a gap control system. The results show that the stable region can be extended by using the reverse gravure roll system.     

Knurl roll design for stable rotogravure coating

When a thin film of fluid is coated on a web by any type roll or spreader coater such that the fluid splits while in a divergent channel the resulting fluid surface is not smooth but is made up of ridges, running in the direction of coating. At production speeds (50 m/min and higher) there is a natura frequency of this fluid ribbing effect which is independent of the type of coater and fluid rheology, and depends only on the depth of the wet coat of transferred liquid. A study has been made to determine how this natural ribbing frequency is related to coating stability on a rotogravure coater with ruling mill knurled coating roll. It has been observed that many knurl rolls deposit a metastable fluid pattern on the web when the resulting fluid sur matches the knurl pattern of the roll. Under some coating conditions this resulting surface becomes unstable and the fluid is transferred to a conditio fewer ridges per cm, resulting in a non-uniform coated surface. It was found that to enhance stable transfer from the knurl grooves it is necessary to the knurl roll surface to match the natural fluid ribbing frequency. The farther the knurl line frequency deviates from the natural fluid ribbing frequ the more often the coating surface will become unstable.     

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

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

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

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

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.     

Fluid absorption during forward roll coating of porous webs

Forward roll coating is a common process to deposit thin liquid films onto a continuous web. When the web is porous, some amount of the fluid is forced into the web in the nip. This removal of fluid, along with the deformation of the backing material, influences transfer in the nip as well as operational issues such as misting and coating defects. While much has been reported on forward roll coating for non-porous webs, little has been done when the web is porous.A laboratory roll coating device is used to characterize the pressure profile, the rubber deformation, and the film thickness as the fluid is in contact with a porous web. A pressure transducer is used to record the pressure profile in the nip. The film thickness on the steel roll surface and the gap between the rolls are measured with capacitance probes. Silicone oils, with three different viscosities, are used as test fluids. Three different papers are used in these tests. A model is proposed to describe the pressure profile, rubber deformation, and absorption in a forward roll coating device. The differential equations are solved to describe the nip behavior. A simplified model is also proposed, using an average nip pressure and Darcy's law, to predict penetration in the nip. The proposed models compare well with the experimental results and predict the dependence on viscosity, nip load, and paper permeability. The experimental results with low viscosity fluids show some speed dependence that is not captured by the model, but the high viscosity fluid behavior agrees well with model predictions.     

Coating with deformable rolls: An experimental investigation of the ribbing instability

Results from a systematic experimental investigation of deformable roll coating, operating in forward mode, are presented, with particular emphasis as to how the governing parameters of the coater (peripheral roll speed and gap setting) and the physical properties of the associated deformable roll covering (thickness, hardness and material type) influence the onset of the ribbing instability. It is found that for positive gap settings, the critical roll speed at which ribs are first observed can be increased by employing a larger gap or using a thicker covering of compliant material. For negative gap settings, explored here for the first time, ribbing is observed to be ever present over the operating range considered. However, for large negative gaps, a reduction in roll speed and the use of a thinner compliant covering are all found to produce ribs of shorter wavelength.     

In situ measurement of water at the organic coating/substrate interface

In situ and quantitative information on the water layer at the organic coating/substrate interface is crucial for understanding and preventing the failure of organic coating systems. A technique, based on a two-layer model derived rigorously from internal reflection theory, has been developed for measuring in situ the thickness and amount of the water layer at the organic coating/substrate interface. The technique gives new insight into the processes by which water degrades the coating/substrate bonds. In this technique, a transparent or an opaque organic coating of sufficient thickness is applied to an internal reflection element (IRE) with or without a thin metallic film, which is used as the substrate. A water chamber is attached to the organic-coated specimen. After adding water to the chamber, Fourier transform infrared-multiple internal reflection (FTIR-MIR) spectra are taken automatically at specified time intervals without disturbing the specimens or the instrument. Water uptake in the coating and FTIR-MIR spectra of water on the coating-free substrate are also used for the analysis. Examples of clear and pigmented coatings on untreated and treated substrate surfaces are given to demonstrate the technique. Results of water accumulation at the coating/iron interface with and without applied electrical potentials are given. In addition to measuring water at the coating/substrate interface, the technique provides a means for studying the transport of water through a coating adhered to a substrate. Information on water at the interface and its transport properties through coatings applied to a substrate is valuable for interpreting corrosion, blistering and delamination of organic coating systems, and for developing models for use in predicting the serivce lives of protective coatings.     

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

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

一种涉及弹性流体动力学的精密涂布系统

一种精密涂布系统,包括张力控制辊涂布(TWRC)和张力控制条缝涂布(TWSC),这种系统主要应用于薄层涂布领域,比如影像行业磁记录介质、LCD光学薄膜防反射膜、防眩光以及硬化膜、柔性敏化太阳能电池领域导电薄膜等等。此系统涉及一种弹性流体动力学,当物理系统同时牵涉流体流动所造成流体摩擦力与可变形固体所产生的弹性应力,且可变形固体成为流体流动的边界,此种相互作用的系统称为弹性流体动力系统(Elastohydrodynamic)简写EHD。     

Novel-stratified low reflectivity anti-reflective coatings derived from self-assembly of high refractive index nanoparticles

We describe new anti-reflective multilayer optical coatings formed from a single liquid coating solution in a single coating step. The nanoparticles in these coatings self-organize as the liquid coating dries on the substrate. The self-assembly of the nanoparticles results in the formation of a two-layer dielectric stack composed of alternating high and low refractive index layers. For the first time, using this process, an ultra-low reflectivity film can be inexpensively and simply formed using high refractive index titanium oxide nanoparticles and a low refractive index fluoropolymer. These films can be made scratch or abrasion resistant by the inclusion of silane coupling agents and low refractive index nanoparticles.     

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.     

Effect of IR transparency of the top polymer layer on low thermal IR emittance paint coatings

A two-layer model was developed to accurately represent low thermal IR emittance paint coatings. The coatings were formed by mixing aluminum flakes with polymeric binders. Because of the different densities of these two components, a two-layer structure was constructed with a pure polymeric layer on top and an in-homogeneous layer underneath. The absorption properties of the top polymeric layer hinder the IR reflectance of the coating. This two-layer model was used to quantitatively determine how IR transparency of the top polymer layer influences the reflectance of the coating. The bottom layer was considered to be a substrate with a specific diffuse reflectance, and the ray tracing method was used to calculate the reflectance of this polymer-coated, diffusing substrate system. The reflectance of the coating with a low absorption coefficient (0.01 μm^?1) top polymer (3 μm thick) was reduced 50% compared with when a high absorption coefficient (0.1 μm^?1) top polymer was used. Measurements of coatings of chlorinated rubber and alkyd rubber on a diffuse gold substrate confirmed the reliability of the model predictions.     

Investigation of interfacial instabilities with a two-layer slide coating process

Organic electronics have been thoroughly investigated due to their broad application potential, ranging from light-emitting diodes to photovoltaics. The processing of organic electronics is trending from vacuum toward wet chemical deposition, which allows fast low-cost mass production of devices with scalable dimension. One of the current challenges of wet film processing is the redissolution of already dried active materials when applying a liquid top layer. Further, increasing overall process efficiency by coating multiple liquid layers in one step raises such challenges as liquid–liquid mixing or dewetting. This article describes the experimental investigation of these instabilities for two-layer flows with organic solvents. A modified slide coating device was chosen where an extended plate after the slot exit allows prolonged observation of the flow while it travels down the plate. During experimentation, stable and unstable two-layer flows as well as different types of instabilities were detected. The key finding is a correlation of flow stability with the spreading coefficient, a combined measure of surface and interfacial tensions. Focusing on fluid properties, this paper succeeds in defining a three-dimensional stability window for a dual-layer flow.     

Tribomechanical and microstructural properties of cathodic arc-deposited ternary nitride coatings

Sintered carbides are promising materials for surfaces that are exposed to extreme wear. Owing to their high service load, ceramic-based thin films are coated on carbides using different techniques. In this study, non-toxic and cobalt-free powder metallurgy-sintered carbide samples were coated with TiN, TiAlN, CrAlN, and TiSiN ceramic-based thin film coatings by cathodic arc physical vapor deposition. The microstructure (phase formation, coating thickness, surface roughness, and topography), mechanical properties (hardness, modulus of elasticity, and plasticity indices), and tribological properties (nanoscratch and wear behavior) of the thin film coatings were investigated. No cracks or defects were detected in these layers. The ceramic-based ternary nitride thin film coatings exhibited better mechanical performance than the TiN coating. The TiN thin film coating had the highest average surface roughness, which deteriorated its tribological performance. The ternary nitride thin film coatings exhibited high toughness, while the TiN thin film coating exhibited brittle behavior under applied loads when subjected to nanoscratch tests. The wear resistance of the ternary nitride coatings increased by nearly 9–17 times as compared to that of the TiN coating and substrate. Among all the samples investigated, the substrate showed the highest coefficient of friction (COF), while the TiSiN coating exhibited the lowest COF. The TiSiN thin film coating showed improved mechanical and tribological properties as compared to other binary and ternary nitride thin film coatings.     

预涂卷材涂料涂装工艺对涂膜性能的影响

介绍了预涂卷材涂料涂装时,前处理辊涂方式、涂装线的线速、炉温、板温(PMT)及膜厚等涂装参数对涂膜性能的影响。     

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

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

微凹版涂布弊病的因果分析和解决对策

目前,随着平面显示器尤其是液晶显示器(LCD)工业的飞速发展,每一个液晶显示器的面板都包括了如下部分:彩色过滤器、偏光片、背光单元和IC驱动。这些部件的很多部分是通过不同的涂布方法来生产的。例如,光学薄膜防反射、防眩光和硬涂层保护薄膜。这些功能薄膜都需要非常高的涂层均匀性,譬如细竖道、竖条道、肋骨棱、空气夹带等等这些涂层缺陷是不允许的,并且涂层湿厚度必须在几个微米之间,干厚在几百个纳米之间,能够生产出涂层厚度在微米大小的功能薄膜的常规方法包括了凹版和微凹版涂布方法。本文介绍了微凹版涂布原理以及涂布工艺,结合实践总结微凹版涂布弊病现象,分析了微凹版涂布过程中易出现的涂布弊病原因,并提出了解决微凹版涂布弊病的方法,在微凹版涂布试验和放量生产过程中,采用这些方法来解决出现的微凹版涂布弊病问题,具有良好效果。     

A Mathematical Model for the Design and Fabrication of Polymer Solar Cells by Spray Coating

Spraying of solution-processable materials such as organic molecules, polymers, nanoparticles, and quantum dots is a viable candidate for the coating process and fabrication of thin film solar cells and other similar semiconductor devices. Spray coating, similar to spray painting in the automotive industry, is a fast process and can be scaled up and used for the roll-to-roll fabrication of solar panels. In this paper, attempts are made to understand various steps of the process and develop a simple model as a design tool. The model equations are solved numerically for the spray coating of a P3HT-PCBM active layer in a polymer solar cell using ultrasonic atomization to investigate the effect of process parameters on the thin film characteristics, such as the film thickness and heat consumption needed to vaporize the solvent. It is concluded that when using spray coating with a small thermal budget, large areas with desired submicron- and nanometer-sized thicknesses can be made in a fast process. Cost of thermal energy and materials decreases with an increase in the substrate speed and nozzle-substrate distance.     

Development of multiple-droplet drop-casting method for the fabrication of coatings and thin solid films

Spray coating is a commercial and low-cost technique for the fabrication of large-area coatings and thin films, but it is a stochastic process that is hard to control, as far as the fabrication of thin coatings and solid films is concerned. On the other hand, drop-casting is a facile and more controllable coating technique than spray coating, but its application is limited to small-area thin solid films and coatings. The objective of this work is, therefore, to study the feasibility of impinging an array of droplets, rather than just one droplet, to fabricate polymeric and other solution-processed thin films with larger surface areas than those produced by conventional drop-casting. To this end, in this study, four droplets of poly(3,4-ethylenedioxythiophene)–polystyrene sulfonate (PEDOT:PSS) solution are released simultaneously and impinged on the four vertices of a square on a wettable solid surface to make a thin film. The effect of the substrate texture on the spreading and the film formation process is studied. As a novel idea, the substrate is excited by ultrasonic vibration to improve the droplet spreading and coalescence. It is shown that as time elapses, the impinged droplets successfully coalesce and make a thin film. Surface morphology and roughness of the resulting PEDOT:PSS thin solid films show that, except on the edges, the resulting thin solid films are uniform. This leads us to conclude that the application of equal-sized and equally-spaced multiple droplets released simultaneously and impinged on vibrating substrates could be considered as a new coating technique, which has some of the benefits of the spray coating, but it is much more controllable than spray coating.