Web Speeds Research Articles

Contact line dynamics in curtain coating of non-Newtonian liquids

The rheological characteristics of liquids play an important role in the flow near dynamic contact lines, where the deformation rates are extremely large. The liquid contact line dynamics and the free surface configuration in the curtain coating of polymer solutions were experimentally studied using visualization to determine the effect of two rheological characteristics: shear thinning and extensional thickening. The critical web speeds for heel formation, where the contact line on the moving substrate shifts upstream of the falling curtain, and air entrainment, where the contact line shifts downstream and air bubbles appear, were determined over a range of flow rates. The critical conditions were compared to the behavior observed for a Newtonian liquid. Moreover, the contact line dynamics were described by three dimensionless parameters: the Deborah number, the Ohnesorge number, and the ratio of the web speed to the liquid curtain velocity at the contact line.

A Heat Transfer Model for Graphene Deposition on Ni and Cu Foils in a Roll-to-Roll Plasma Chemical Vapor Deposition System

Abstract High-throughput production is a major bottleneck for integration of graphene-based technologies in existing and future applications. Here, a semi-empirical heat transfer model is developed to optimize large-scale deposition of graphene on Ni and Cu foils in a roll-to-roll (R2R) plasma chemical vapor deposition (CVD) system. Temperature distributions in Ni and Cu foils during deposition are recorded with in situ temperature measurements using near-IR optical emission spectroscopy. The model indicates that foil movement significantly affects the temperature distribution and the cooling rate of the foil. Consequently, graphene growth on Cu is limited to lower web speeds for which the foil temperature is higher, and the residence time in the plasma is longer. On the other hand, graphene can be deposited on Ni at relatively higher web speeds due to moderately high diffusion rate of carbon in Ni and increased cooling rates up to 20 K/s with higher web speed. Critical limitations in the production rates of graphene using R2R CVD process exist due to significant effects of web speed on the temperature distribution of the substrate. The thermal analysis approach reported here is expected to aid in enhancing the throughput of graphene production in R2R CVD systems.

The fluid mechanics of tensioned web roll coating

Tensioned web-roll coating is widely used but has surprisingly received little research attention. Here, a new semi-empirical model that predicts film transfer from applicator roller to web is developed and tested against data collected from a pilot coating line. The film transfer is found to vary linearly with web to applicator speed ratio S. Flow stability investigations revealed three types of defects: rivulets, air entrainment due to dynamic wetting failure and cascade, occurring at different values of S and capillary number Ca. Rivulets occurred at Ca < 0.4 and S > 0.71–0.81, air entrainment at Ca > 0.4 and S > 0.71–0.83 and cascades at S > 1.1 for Ca up to 6. Web speeds at which dynamic wetting failure occurred were, for the same Ca, comparatively higher than those that occur in dip coating. The data show that such hydrodynamic assistance is due to the coating bead being confined, more so with increasing web wrap angle β.

Photonic Curing of Nickel Oxide Transport Layer and Perovskite Active Layer for Flexible Perovskite Solar Cells: A Path Towards High-Throughput Manufacturing

High-throughput roll-to-roll (R2R) manufacturing of perovskite solar cells (PSCs) is currently limited by thermal processes that take tens of minutes each, translating to impractically long annealing tools at high web speeds. In addition, PSCs are usually made with metal oxide transport layer materials that require high temperatures for thermal annealing. Here, we demonstrate the fabrication of PSCs using photonic curing, instead of thermal annealing, to convert NiOx directly from sol-gel precursors for hole transport layers and to crystallize methylammonium lead iodide (MAPbI3) active layers on flexible Willow® Glass substrates. Photonic curing uses short, intense pulses of light to process materials at a high speed, hence it is compatible with R2R manufacturing. We achieved power conversion efficiencies (PCEs) of 11.7% in forward-scan and 10.9% in reverse-scan for PSCs made with photonic cured NiOx and MAPbI3 films. Furthermore, both NiOx and MAPbI3 films could be processed with a single photonic curing pulse, with a web speed of 5.7 m/min, and still produce PCEs comparable to thermally annealed control samples. Based on the single-pulse photonic curing condition for each film, we project a web speed of 26 m/min, laying a pathway to high-throughput production of perovskite solar modules.

Roll-to-roll micromolding of UV curable coatings

In this study, fast and continuous fabrication of microscale structures by roll-to-roll UV imprinting or micromolding is demonstrated on a 121 mm wide web. This process is enabled by a UV curable thiol-ene-acrylate resin system, following the work of Stadlober and coworkers. A series of formulations were prepared with fast curing speeds at ambient conditions, low viscosities, and tunable mechanical properties. The rate and extent of curing as a function of the formulation composition were investigated with Fourier transform infrared spectroscopy. Consistent with the past work, we show that the thiol-ene-acrylate formulations reached high double bond conversions (> 80%) with the maximum conversion increasing with the relative thiol content and decreasing with the viscous urethane acrylate oligomer content. The double bond conversion of the patterned coatings (in contact with the mold) is shown to be ~ 80% for a range of UV lamp powers and web speeds up to 2.7 cm/s. Areas of the coating not covered by the mold required higher UV lamp power and/or lower web speeds to cure to a tack-free state. Microscale channels and arrays of recessed wells with various dimensions and pattern densities were continuously fabricated. Our findings show the successful use of a tetrafunctional thiol in the thiol-ene-acrylate resin system. We also discuss guidelines for the selection of processing conditions for the manufacturing of structured plastic substrates using roll-to-roll imprinting processes, opening up potential new applications.

Delaying breakup and avoiding air entrainment in curtain coating using a two-layer liquid structure

Successful curtain coating is limited by two flow instabilities, the breakup of the liquid curtain below a critical flow rate and air entrainment above a maximum substrate speed. These limitations make thin film coating at high speed a great challenge. Previous work has shown that the curtain breakup can be delayed to lower flow rates if the flowing liquid has viscoelastic behavior. However, viscoelastic stresses near the dynamic contact line destabilize the flow in that region, leading to the onset of air entrainment at lower web speeds. We propose and experimentally validate the use of a two-layer curtain coating configuration, with a shear-thinning liquid as the bottom layer and a viscoelastic liquid as the top layer, that expands the processing window and enables thin coating at high speeds.

Operational limits of blade coating associated with high aspect ratio pigments: Part I—bench top blade coater

Operational issues, such as stalagmite formations, scratches, or spits, develop during blade coating of high solids coatings at high web speeds. Coatings that contain high aspect ratio pigments exhibit these difficulties at lower solids concentrations and slower machine speeds than coatings comprised of more spherical shaped pigments. There are a number of potential reasons behind this phenomenon, but a clear mechanism is not well established. Using a bench top blade coater without a base sheet, the goal of this study is to understand the operational limits that occur due to the coating suspension interacting with the blade. Pigment suspensions are applied in excess in front of the blade. A camera mounted near the blade exit is used to detect defects and buildup on the backside of the blade. The machine speed at which deposits on the blade first appear was determined for three pigments having different shape factors and at varying weight fractions. A Newtonian fluid was also used in the coating device. A runnability window based on shape factor and solids content was developed. Small changes in the solids concentration displayed changes in the speed at which blade deposits begin to appear. No operational issues could be produced using the Newtonian fluid. Presence of the particles at solids concentration nearing their immobilization solids mustbe contributing to the development of runnability issues.

Operational limits of blade coating associated with high aspect ratio pigments: Part II—cylindrical laboratory coater

Process issues such as blade deposits, scratches, spits, and defects on the coating surface develop during blade coating of paper at high solids content and fast machine speeds. Coatings that contain high aspect ratio pigments display these issues at lower solids concentrations and slower web speeds than spherical or block shaped pigments. The shape of the pigment particles influences the characteristics of the coating formulation after drying, and high aspect ratio pigments can lead to high gloss surfaces that have good printing properties and are of interest in barrier coatings. The rheology, particle packing, and water retention are all functions of the pigment shape and size distribution. The base sheet properties of porosity, surface roughness, and permeability interact with the coating and the blade coater operating conditions. The possible causes of poor runnability are part of an interrelated and complex system. A cylindrical laboratory coater (CLC) was utilized to determine the speed at which operational and quality issues begin at varying solids concentration for two pigments with different shape factors on two different paper surfaces with varying absorption rates. An operational window was developed for both pigments. Characteristics of the pigment suspensions were explored in order to find a relationship with the onset of operational problems. The high shear rate viscosity of these suspensions did not explain the operational windows. The difference between the solids concentration and the immobilization solids, which is a function of the shape factor, has a linear relationship to the runnability limits. This could be ascribed to the tight packing structure of the particles as the solids content approaches the immobilization solids.

Toward high volume solution based roll-to-roll processing of OLEDs

The large volume production of flexible electronics by solution based roll-to-roll (R2R) manufacturing technologies is a promising upscaling strategy for the organic electronics industry. Typical optoelectronic devices like organic light emitting diodes (OLEDs) consist of a complex stack of functional layers. Solution deposition of these structures eliminates the need for expensive vacuum processing. This contribution presents approaches for solution based R2R production methods of functional OLED layers on flexible polymer substrates. The development of a R2R line with two slot-die coating stations is discussed which can deposit two uniform layers consecutively in a single run (“tandem coating”) at web speeds up to 30 m/min. Furthermore, it offers the unique feature that there is no contact between the rollers and the top side of the substrate where the functional coating is deposited. Thereby, an important source of particle contamination and other damage to the device is eliminated. In addition to continuous deposition, stripe and intermittent coating techniques have been developed, allowing the production of patterned layers. Finally, examples will be shown of OLEDs where two functional materials are deposited by R2R processing from solution.

Large-Area Nanoparticle Films by Continuous Automated Langmuir-Blodgett Assembly and Deposition.

The operating parameters and resulting surface morphology of automated Langmuir-Blodgett deposition of monosized micrometer-scale silica colloids from an aqueous suspension are investigated. This apparatus allows continuous roll-to-roll deposition of particles into well-ordered arrays. The reproducible deposition of particle monolayers at low to moderate deposition rates at web speeds of less than 10 mm/s is possible and accurately characterized by a simple mass balance of particles deposited from solution. At faster deposition rates, Landau-Levich flow increases the film thickness such that flow instabilities hinder uniform particle deposition. A simple phase diagram outlines transitions from dispersed to multilayer coatings and from uniform to erratic deposition patterns. While the threshold of maximum deposition rate is well-defined for these conditions, changing operating parameters, particle size, and fluid viscosity and evaporation rate, the maximum speed can be increased significantly.

Output Regulation of Nonlinear Systems With Application to Roll-to-Roll Manufacturing Systems

This paper deals with the problem of synthesizing feedforward control to aid the regulation of output of a nonlinear system in the presence of partially known exogenous inputs. The problem appears in many engineering applications including Roll-to-Roll (R2R) manufacturing systems. Currently known methods for this problem either require the solution of a constrained partial differential equation or the preview information of the signal to be tracked. The novelty of this paper lies in synthesizing feedforward control as the solution of a system of differential-algebraic equations, which is considerably less complex and suitable for practical implementation. In this paper, we consider the problem of regulating the output while rejecting the disturbances and apply it to R2R manufacturing systems. The problem of tracking reference signals can also be handled with the suggested technique. We assume that the disturbance signal is the output of a known exogenous system with unknown initial conditions. A parameter identification scheme to estimate the unknown initial conditions is developed. The proposed technique is successfully applied to control of web tension in a large R2R machine which mimics most of the features of industrial R2R machines and contains real-time hardware and software that is used in industrial practice. Extensive experiments were conducted to evaluate the proposed scheme for web tension control under various experimental conditions, including different web speeds and materials. We will present and discuss the representative experimental results with the proposed technique and provide a comparison with an industrial PI control scheme to highlight the benefits of using the proposed scheme.

Recent Progress on Printed Flexible Batteries: Mechanical Challenges, Printing Technologies, and Future Prospects

AbstractTraditional printing methods offer the advantage of well‐matured technology, high accuracy of depositing inks over flexible substrates at high web speeds, and low cost of fabrication. The components of a battery—the current collectors, active layers, and separator—can all be deposited using conventional printing techniques by designing suitable inks. A combination of low thickness of printed electrodes, flexible packaging, battery architecture, and material properties makes printed batteries flexible. In this paper, we will discuss material challenges and mechanical limits of flexible printed batteries. We will review several printing techniques and present examples of batteries printed using these methods. In addition, we will briefly discuss other novel non‐printed compliant batteries that have unique mechanical form.

Inkjetable and photo-curable resists for large-area and high-throughput roll-to-roll nanoimprint lithography

We report on our development strategy of photo-curable resists for nanoimprint lithography (NIL) based on modularity. Starting with a basic formulation, we address two topics: the integration of fluorinated additives and the enhancement of the dry etching stability. We prove both concepts by the introduction of two different resists derived from the same basic formulation. The viscosity of the novel resist materials was optimized for inkjet dispensing at room temperature (RT). The novel resist materials can be applied either in NIL batch processes or in high-throughput roller processes. Batch-wise imprints were performed on various substrates such as Si or plastics, demonstrating the distinctive application versatility of the novel materials. Dry etching of spin-coated thin films on Si wafers was performed, demonstrating an etch stability versus Si of 3.5:1 by using the resist formulation with improved etching stability. Roll-to-roll NIL at high throughput on large areas was performed with web speeds of up to 30 m min−1 with different stamp materials. We conclude that all resists reported herein can be deposited via inkjet dispensing at RT, are suitable for continuous high-throughput imprinting on flexible substrates, and are applicable in step-wise NIL processes with good etch resistance in dry etch processes.

Generic roll‐to‐roll compatible method for insolubilizing and stabilizing conjugated active layers based on low energy electron irradiation

ABSTRACTIrradiation of organic multilayer films is demonstrated as a powerful method to improve several properties of polymer thin films and devices derived from them. The chemical cross‐linking that is the direct result of the irradiation with ∼100 keV electrons is fast and has a penetration power compatible with thin plastic foils of one to two hundreds of microns typical of devices explored in organic electronics. We demonstrate here that active layers and complete devices can be subjected to electron irradiation‐induced cross‐linking thus facilitating multilayer solvent processing and morphological stability. The method is fast, generic, contactless, and fully compatible with high‐speed roll‐to‐roll processing of i.e. polymer solar cells at web speeds in excess of 60 m min−1. We employ fully printed, flexible, and foil‐based indium‐tin‐oxide free polymer solar cells in this study to demonstrate the technique. We also demonstrate that polymer solar cells are exceptionally stable towards ionizing radiation and find that doses as high as 100 kGy can be used before any significant decrease in performance is observed. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40795.Together with Mokarian‐Tabari et al., J. Appl. Polym. Sci. (2014) 131, 40798, doi:10.1002/app.40798, this article is part of a Special Issue on Polymers for Microelectronics. The remaining articles appear in J. Appl. Polym. Sci. (2014) volume 131, issue 24. This note was added on 1st July 2014.

Fast Inline Roll‐to‐Roll Printing for Indium‐Tin‐Oxide‐Free Polymer Solar Cells Using Automatic Registration

AbstractFast inline roll‐to‐roll printing and coating on polyethylene terephthalate (PET) and barrier foil was demonstrated under ambient conditions at web speeds of 10 m min−1 for the manufacture of indium‐tin‐oxide‐free (ITO‐free) polymer solar cells comprising a 6‐layer stack: silver‐grid/PEDOT:PSS/ZnO/P3HT:PCBM/PEDOT:PSS/silver‐grid. The first and second layers were printed at the same time using inline processing at a web speed of 10 m min−1 where flexographic printing of a hexagonal silver grid comprises the first layer followed by rotary‐screen printing of a PEDOT:PSS electrode as the second layer. The third and fourth layers were slot‐die coated at the same time again using inline processing at a web speed of 10 m min−1 of firstly zinc oxide as the electron transport layer followed by P3HT:PCBM as the active layer. The first three layers (silver‐grid/PEDOT:PSS/ZnO) comprise a generally applicable ITO‐free, semitransparent, electron‐selective front electrode for inverted polymer solar cells. This electrode shows a low sheet resistance (∼10 Ω/□) and good optical transmission in the visible range (∼60 %). The solar cell stack was completed by rotary‐screen printing of a hole‐collecting PEDOT:PSS layer at 2 m min−1 and a comb‐patterned silver‐grid back electrode at the same speed. The solar cells were post processed by using fast roll‐to‐roll switching to a functional state.

High rate roll to roll atomic layer deposition, and its application to moisture barriers on polymer films

Atomic layer deposition has been shown to provide high quality single layer moisture barrier films on polymer substrates, but conventional pulse-based processes are too slow to be commercially feasible. One way to overcome this speed limitation is to avoid the need to pulse and purge precursors by moving the substrate between zones containing the precursors, passing through intermediate purge zones. Recently, several groups have reported various approaches to accomplishing this, including the approach discussed here in which the flexible web is passed between precursor zones in a serpentine pattern. Al2O3 and TiO2 barrier films 12 to 20 nm thick with water vapor transmission rates in the range of 10−4 g/m2/day have been demonstrated for web speeds in excess of 1 m/s on 100 mm wide polyethylene terephthalate web. Scale-up of this process to 300 mm wide web in a system capable of depositing 10–20 nm of film in a single pass is currently under way. This scale-up effort and the potential for very high volume, low cost moisture barrier production utilizing this technique are discussed.

Roll-to-roll manufacture of pentacene-based thin film transistors with a flash-evaporated polymer dielectric cured with an e-beam

ABSTRACTWe report our fabrication of pentacene field effect transistors (FETs) based on a vacuum processed and e-beam cured polymer electrolyte (i.e., tripropylene glycol diacrylate (TRPGDA)) as a gate dielectric layer on flexible wide web substrates. The deposition of the semiconductor and gate insulator layers is carried out in Oxford's roll-to-roll vacuum web processing facility and could be combined in-line with roll-to-roll pattern metallization. The aim of the work is to demonstrate the ability to create all-evaporated transistors, exploiting the kind of technologies at present extensively used in the food packaging industry, for example, in which all layers can be deposited at high web speeds. Ours is a room temperature and a solvent-free method with an ultrahigh deposition rate (web speeds in excess of 100 m/min are possible).The performance of the pentacene and the polymer dielectric materials within devices was investigated, demonstrating (1) the ability to deposit good quality pentacene layers in the roll-to-roll environment and (2) the importance of the e-beam curing conditions of the dielectric layer on the performance of the organic FET devices. This deposition route creates a smooth, pin-hole-free dielectric layer as it is deposited onto the substrate as a monomeric liquid, before curing, and there is no mass-loss such as in solvent-based deposition processes. These devices have a 250 μm channel length and an aspect ratio of 16. No self-assembled monolayer (SAM) or other surface modification had been applied to the insulator layer to achieve these properties.The tuning of the transistors' operating voltage and output characteristics was feasible through the ease of control of the polymer dielectric thickness achieving a threshold voltage of less than 10 V. On/off ratio in excess of 103, and mobility = 0.1 cm2V-1s-1 have been achieved.The ability to integrate such a high deposition rate polymer process, into a single step, multilayer, vacuum deposition process with conventional vacuum deposition sources provides a possible route to low cost and large area electronic device processing.

A Breakthrough High Speed Wide Format Print Concept for Textiles

Digital printing has been around in the industry for many years but is still contributes a relatively small volume of printed substrates compared to analogue printing, especially in the textile industry. With the introduction of new industrial inkjet printing technologies and processes, and with recent advances in nanotechnology and in understanding the interaction between inks, print heads and substrates, inkjet technology will gradually move into new areas of application beyond graphics printing and into functional printing, material deposition and additive manufacturing. Functional printing and material deposition for textiles have been demonstrated in the laboratory as proofs of concept or prototypes and will see a breakthrough into the industrial environment in the coming decade. However graphics printing is breaking into the industrial market right now in niche applications and replacing analogue processes.A significant breakthrough for industrial inkjet printing and functional printing in the textile industry, that is currently dominated by analogue technologies, depends on the availability of reliable and scalable industrial inkjet printing systems and processes with competitive costs per printed area and the availability of industrial inks and finishes at prices that allow high productivity at a competitive cost per printed area compared to screen printing.This paper will focus on the recent results of development projects involving TenCate, Xennia and Reggiani where great advances have been made in the development of novel inkjet processes and technologies, enabling the use of inkjet printing in textile finishing processes, especially in the area of wide format printing and coating applications.The breakthrough development is a novel and patented industrial print concept where both the substrate and the print head carriages move continuously without a step and scan phase, leading to a continuous diagonal bi-directional print process with significantly higher throughput than traditional step and scan systems. The concept is scalable for width and speed width and enables web speeds up to 20m/min. The diagonal build-up of the image leads to high reliability and built in nozzle redundancy. The basics behind the continuous print process can be transferred to other wide format print applications beyond the textile industry.Next to the novel bi-directional continuous print technology other technologies will be reviewed that enable a breakthrough in the wide format printing and functional printing market. With recent developments in the area of textile finishing technologies based on digital printing (coating, dyeing and functional finishes) and with new inks and functional materials for digital textile printing it is expected that in the coming years digital printing will be able to replace analogue processes in textile printing, textile finishing and textile coating but also conquer new niche markets and industrial applications.

Growth of pentacene thin films by in-line organic vapor phase deposition

We present the extension of the organic vapor phase deposition technique to an in-line geometry, in which the sample travels underneath an elongated showerhead that sprays molecules transported by a stream of carrier gas. Highly uniform pentacene films are grown at web speeds of up to 2.1 m/min, equivalent to an average deposition rate of 105 Å/s in a static system. With transistor mobilities of up to 1.5 cm 2/V s, these pentacene films are of high electrical quality. Importantly, this quality is conserved up to the highest deposition speeds. We discuss the relationships between in-line deposition rate, morphology and crystallinity of the deposited pentacene films and their electrical characteristics.

In-line determination of the thickness of UV-cured coatings on polymer films by NIR spectroscopy

We report on the development of a measuring method based on near-infrared (NIR) spectroscopy, which is able to determine the thickness of UV-cured coatings and which can be used for in-line monitoring in technical coating processes. In particular, acrylate coatings, which were applied to transparent polymer films with a typical thickness of 5–35 μm, were investigated. NIR spectra were recorded in transflection mode. Quantitative analysis of the spectral data was carried out with partial least square (PLS) regression. In-line measurements were performed on a pilot-scale roll coating machine at web speeds up to 50 m/min. It was shown that quantitative data with excellent precision (i.e. with a standard deviation lower than ±1 μm) and high time resolution (2.5 spectra/s) can be obtained.