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.     

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.     

Process limits in two-layer reverse roll transfer

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

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.     

Estimation of film thickness nonuniformity effects on coating optical properties

Film nonuniformities cause many problems for the coating scientist. One area of coating film performance that they impact directly is optical properties. Measurements and predictions of film optical performance are all made assuming uniform film thickness. Incomplete leveling and other film application problems often yield nonuniform films with significant film thickness fluctuations. These film thickness imperfections can be modeled by a sine wave thickness fluctuation, and this fluctuation can be used to estimate the effects of nonuniformity in thickness on optical properties. This has been done using both approximate equations for scattering, transmission, and reflectance in films (zero absorption) and for the Kubelka-Munk equations including absorption. the effects of average film thickness and amplitude of fluctuation on measured S, R, and T values are predicted numerically for several cases of [K, S] pairs consistent with wavelength independent situations (white or similar films) over black substrates, and then the calculations are used to consider colored films [considering δE(L*a*b*)] over black substrates. Film thickness fluctuations are shown to have considerable impact on film optical properties, especially in thin highly scattering films. Examples of the types of predictions that can be made are given; how the results depend on the coating optical parameters is also shown.     

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     

Tri-helical gravure roll coating

A mathematical model is presented and solved for the fluid flow within the coating bead of a tri-helical gravure roll coater, operating in reverse mode. A variety of rolls etched circumferentially with grooves of different cross-sectional shape and aligned at non-zero angles of pitch are investigated. Predictions of fluid pick-out from the grooves are compared with complementary experimental data. Quantitative agreement between the two is found to be very good, showing a linear increase in pickout as a function of web-to-roll speed ratio and groove depths up to the point at which streaking, as observed experimentally, occurs and beyond which the model is no longer valid. In regions of parameter space for which there is no experimental data available for comparison purposes the model predicts that: (i) fluid pick-out decreases with increasing groove depth while the film thickness tends to increase and (ii) an increase in groove aspect ratio leads to a reduction in both pick-out and film thickness for a given land width and groove cross-sectional shape.     

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.     

Thermomechanical analysis and modeling of the extrusion coating process

During the extrusion coating process, a polymer film is extruded through a flat die, stretched in air, and then coated on a substrate (steel sheet in our case) in a laminator consisting of a chill roll and a flexible pressure roll. The nip, i.e. the area formed by the contact between the pressure and the chill rolls, constitutes the heart of the extrusion coating process. Indeed, in this region, some of the most critical properties, such as adhesion, barrier properties, optical properties, are achieved or lost. In this article, we first present an experimental investigation of the coating step, which enables to characterize the leading thermomechanical phenomena. It is shown that there is no polymer macroscopic flow in the nip, but a local flow within the asperities of the steel substrate surface. This microscopic flow, at the interface between the film and the substrate, is slowed by strong cooling conditions in the nip. Several models are then proposed, giving access to the temperature profile through polymer thickness and substrate, the pressure distribution in the nip as well as the behavior of the polymer melt in the nip at the interface with the substrate. POLYM. ENG. SCI., 2008. © 2008 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。     

Synthesis and characterisation of surface gradient thin conversion films on zinc coated steel

Surface gradient layers on hot-dip galvanised steel were synthesised in order to determine the barrier properties and corrosion resistance of thin amorphous conversion coatings as a function of layer thickness and processing time. For this purpose, a dip coating procedure was established that yields well-defined gradient layers. As a model system for conversion film formation on zinc coated steel, a zirconium based bath chemistry was used. The synthesised zirconium oxyhydroxide gradient films were characterised by localised electrochemical techniques, such as Scanning Kelvin Probe (SKP) and electrochemical impedance spectroscopy using an electrochemical capillary cell. Microscopic infrared reflection absorption spectroscopy (μ-FT-IRRAS) measurements and small-spot X-ray photo electron spectroscopy (XPS) were used as complementary surface analytical techniques. The applied analysing techniques provide a spatial resolution of 100-1600 μm. Thereby, a complete variation of thin film properties, such as thickness, barrier properties, corrosion resistance and chemical composition can be measured as function of the time of film growth on a sample with a length of a few centimetres. This approach allows a precise and accurate determination of structure-to-property relationships of thin conversion films. Moreover, it could be shown that a surface gradient film analysis significantly rationalises experimental time and increases the reliability of the experimental results.     

Spin coating of thin and ultrathin polymer films

The spin coating of thin (> 200 nm thick) and ultrathin (< 200 nm thick) polymer films is examined in several solvents of varying volatility over a broad range of polymer solution concentrations and spin speeds. Experimentally measured film thicknesses are compared with a simple model proposed by Bornside, Macosko, and Scriven, which predicts film thickness based on the initial properties of the polymer solution, solvent, and spin speed. This model is found to predict film thickness values within 10% over the entire range of conditions explored, which gave film thicknesses from 10 nm to 33 μ:m. The model underpredicts film thickness for cases in which a very volatile solvent is used or the initial concentration of polymer is high, while overpredicting film thickness for cases in which a low volatility solvent is used or the initial polymer concentration is very low. These deviations are a consequence of how the model decouples fluid flow and solvent evaporation.     

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。     

Mathematical model for thickness of off-center spin-coated polymer films

Off-center spin coating is a method to fabricate thin film on a substrate where the substrate is located at an off-center distance away from the rotating center of the spin coater. Here, a mathematical model to calculate the thickness of a film fabricated by an off-center spin-coating technique was developed and proposed. The model showed that the off-center film thickness was calculable by using four factors—the on-center film thickness, mass fraction of solid in the wet film, length of the substrate in the radial direction, and off-center distance. Simply, the off-center film thickness was inversely proportional to the off-center distance to the exponent of one-third, that is, the further the off-center distance, the thinner the film. The model was verified where the thicknesses of the films calculated by using the model were compared with the experimental values obtained from the off-center spin-coated films of poly(vinylidene fluoride) at various off-center distances. Both the modeled and the experimental data were of the same trend and in a good agreement with each other, indicating the validity of the model. The limitations of the model were also discussed. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48356.     

Bilaterally Microstructured Thin Polydimethylsiloxane Film Production

Thin PDMS films with complex microstructures are used in the manufacturing of dielectric electro active polymer (DEAP) actuators, sensors and generators, to protect the metal electrode from large strains and to assure controlled actuation. The current manufacturing process at Danfoss Polypower A/S produces films with a one-sided microstructured surface only. It would be advantageous to produce a film with both surfaces microstructured, as this increases the film’s performance efficiency. The new technique introduced herein produces bilaterally microstructured film by combining an embossing method with the existing manufacturing process. In employing the new technique, films with microstructures on both surfaces are successfully made with two different liquid silicone rubber (LSR) formulations: 1) pure XLR630 and 2) XLR630 with titanium dioxide (TiO₂). The LSR films (～70 µm) are cast on a carrier web substrate using a coating blade. The carrier web, which has a sinusoidal corrugation with wave height of 7 µm and a wave period of 7 µm on its surface, imparts corrugations to the bottom surface of the film. The elastomer film on the carrier web is preheated to the gel point, where the elastomer film can retain an imprint made on it. The preheated film at gel point is embossed between the rolls of a gravure lab coater, which corrugates the top surface of the film. The films are then heated, in order to cure completely. For the LSR systems used in this process, the optimum conditions for preheating are 110°C for 4–7 s, while for embossing the temperature is 110°C with 25 psi pressure between the rolls at a speed of 1.4 rpm. Scanning electron microscope (SEM) images confirm the formation of microstructures on both the surfaces of the film.     

Multilayer polyimide nanocomposite films synthesis process optimization impact on nanoparticles dispersion and their dielectric performance

The preparation of polyimide (PI)/nanocomposite films and their thickness are a complex process with some ambiguous variables that are involved in the synthesis process. Therefore, it is crucial to understand those variables and reveal the chemistry behind them. Several methods have been probed until an optimal synthesis process was found. A detailed synthesis process optimization is described in this article to understand all variables, which can influence the molecular weight of polyamic acide (PAA) solution, the thickness of films, and nanoparticles dispersion. The spin coating technique was used to control the thickness of single and multilayer PI/nanocomposite films, which reveals that the thickness of the casted PI film depends on several factors, such as the viscosity, molecular weight of the PAA solution, and the spin speed of spin coater. Factors, which can influence the molecular weight of PAA solution, are discussed in detail. After completing the synthesis process, the single and multilayer PI nanocomposite films are characterized experimentally. The results reveal that using a thin layer of nanocomposite on PI film in the form of a multilayer structure improves the nanoparticles dispersion and thereafter its dielectric properties.     

Characterization of non-uniform diamond-like carbon films by spectroscopic ellipsometry

Optical characterization of diamond-like carbon (DLC) films non-uniform in thickness is performed using spectroscopic phase-modulated ellipsometry. This characterization is based on new formulas for the associated ellipsometric parameters of thin films exhibiting a wedge-shaped thickness non-uniformity. These formulas express the associated ellipsometric parameters by means of the density of distribution of local film thickness. The spectral dependences of the optical constants of these non-uniform DLC films are expressed using the dispersion model based on parametrization of density of electronic states. It is shown that this model of the thickness non-uniformity provides a relatively good agreement between the experimental and theoretical data, indicating that the results of the optical characterization of the non-uniform DLC films are close to the correct results. Moreover, it is shown that the model of uniform thin films is unsuitable for the optical characterization of the non-uniform DLC films studied.     

Characterization of antireflective coatings on poly(methyl methacrylate) substrate by different process parameters

The high/low refractive index organic/inorganic antireflective (AR) hybrid polymers were formed using the sol–gel process, in which TiO₂/2‐hydroxyethyl methacrylate (2‐HEMA) (high refractive index hybrid polymer) and SiO₂/2‐HEMA (low refractive index hybrid polymer) two‐layer thin films were formed on a hard coating deposited poly(methyl methacrylate) (HC‐PMMA) substrate by both spin coating and dip coating. The relationship between the process parameters and the optical properties, thickness, porosity, surface morphology, and adhesion was determined. The results show that the reflectance of the two‐layer thin films on HC‐PMMA substrate is less than 0.21% (λ = 550 nm), with good adhesion (5B) and a hardness of up to 4H. In addition, the thickness, porosity, and roughness of the films affect refractive index and the antireflection properties of the AR two‐layered thin film. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Slot die stripe coating of low viscous fluids

Slot die coating is applied to deposit thin and homogenous films in roll-to-roll and sheet-to-sheet applications. The critical step in operation is to choose suitable process parameters within the process window. In this work, we investigate an upper limit for stripe coatings. This maximum film thickness is characterized by stripe merging which needs to be avoided in a stable process. It is shown that the upper limit reduces the process window for stripe coatings to a major extent. As a result, stripe coatings at large coating gaps and low viscosities are only possible for relatively thick films. Explaining the upper limit, a theory of balancing the side pressure in the gap region in the cross-web direction has been developed.     

Thickness-Dependent Mechanical Behaviour of Aluminium-Coated Polyethylene Terephthalate (PET) Films

Mechanical behaviour of aluminium-coated PET films has been investigated through Dynamic Mechanical Analyser. Aluminium is coated on PET substrate by vacuum thermal evaporation method. As thickness of aluminium coating increases from 150 nm to 350 nm, tensile strength decreases from 108.88 MPa to 99.25 MPa. This mechanical behaviour is correlated with microstructure and its evolution with the thickness of aluminium coating. Al-PET film consists of fine globular grains and average grain size increases monotonically with the film thickness. The relative contribution of the grain size to the strength of aluminium thin films is in good agreement with Hall-Patch equation.