Coating and Drying Defects

Gutoff, Edgar B.; Cohen, Edward D.

doi:10.1002/0470044136

Cited by 81 publications

(53 citation statements)

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“…The top layer has to spread over all the other layers and therefore has to have the lowest surface tension of all the liquids. 7 In multilayer coating of miscible liquids, there is no interfacial tension between the layers. Thus, as far as spreading is concerned, the bottom layer liquids may have any value of surface tension, as long as the top layer has the lowest surface tension.…”

Section: Introductionmentioning

confidence: 99%

Simultaneous multilayer coating of water-based and alcohol-based solutions

2015

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Multilayer continuous liquid coating is the most efficient way to manufacture films that require more than one layer for optimal performance. Twolayer slot coating is one of the different coating methods largely used to deposit two thin, uniform liquid layers onto a moving substrate. In recent years, the operating conditions and solution properties which are required to avoid various coating defects have been discussed in the literature. However, these past studies have not focused on the problem of top layer dewetting, which may occur at some operating conditions. It is important to include in the operating window of the process, the region of the parameter space at which the top layer dewetting is avoided. In this work, we used water-based and alcohol-based solutions to create twolayer films over a moving substrate using a two-layer slot coating die. The critical operating condition at which top layer dewetting is observed is determined experimentally for different liquid viscosities, surface tensions, and wet thicknesses. A physical mechanism that explains the reported phenomenon is proposed. The results show how dewetting can be avoided by controlling the liquid properties (surface tension and viscosity) and wet thickness of each layer and will certainly help the design of two-layer slot coating processes.

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Section: Introductionmentioning

confidence: 99%

Simultaneous multilayer coating of water-based and alcohol-based solutions

2015

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“…The current targets for this technological challenge are low viscosity coating solutions (<100mPa.s), a wet film thickness in the range 0.5-10μm and a minimum speed of 10m/min (Søndergaard et al, 2012). Under such thin liquid film conditions, flow instabilities will occur above a critical coating speed, causing the film to be marred with defects, some subtle, such as uniform ribs across the film or entrained tiny air bubbles and others more pronounced such as lanes of no flow (rivulets) across the film (Gutoff and Cohen, 2006). All these instabilities are undesirable as they limit the performance of the products.…”

Section: Introductionmentioning

confidence: 99%

Slot coating minimum film thickness in air and in rarefied helium

Benkreira

Ikin

2016

Chemical Engineering Science

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This study assesses experimentally the role of gas viscosity in controlling the minimum film thickness in slot coating in both the slot over roll and tensioned web modes. The minimum film thickness here is defined with respect to the onset of air entrainment rather than rivulets, the reason being that rivulets are an extreme form of instabilities occurring at much higher speeds. The gas viscosity effects are simulated experimentally by encasing the coaters in a sealed gas chamber in which various gases can be admitted. An appropriate choice of two gases was used to compare performances: air at atmospheric pressure and helium at sub-ambient pressure (25mbar), which we establish has a significantly lower "thin film" viscosity than atmospheric air. A capacitance sensor was used to continuously measure the film thickness on the web, which was ramped up in speed at a fixed acceleration whilst visualizations of the film stability were recorded through a viewing port in the chamber. The data collected show clearly that by coating in rarefied helium rather that atmospheric air we can reduce the minimum film thickness or air/gas entrainment low-flow limit. We attribute this widening of the stable coating window to the enhancement of dynamic wetting that results when the thin film gas viscosity is reduced. These results have evident practical significance for slot coating, the coating method of choice in many new technological applications, but it is their fundamental merit which is new and one that should be followed with further data and theoretical underpinning.

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“…Recently, this method has been extended to coat large pieces of glass 2 or has been modified for very thin coating of optical films 3 for liquid crystal display (LCD) industries. Detailed introduction of slot die coating can be found in several text books of Kistler and Schweizer 4 and Gutoff and Cohen 5 . The most important feature of the slot die coating operation is the minimum wet thickness at a given coating condition.…”

Section: Introduction Smentioning

confidence: 99%

Operating windows of slot die coating: Comparison of theoretical predictions with experimental observations

et al. 2010

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ABSTRACT:The objective of the present study is to examine the validity of the theoretical predictions on the operating windows of slot die coating. The operating window is defined as a domain inside which stable and uniform coating is possible; different types of coating defects are found outside the window. A flow visualization technique was applied to observe the coating bead, particularly the positions and shapes of the downstream and upstream menisci, just before and after coating defects appear at high coating speeds. Special features of coating bead shapes, which lead to onsets of ribbing and air entrainment, were identified. The two-dimensional flow in the coating bead region was computed by the commercial software package FLOW-3D ® . Critical features observed experimentally for the onset of coating defects were used to judge whether the coating flow was within the operating window. The theoretically evaluated operating windows were found to be much larger than

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Coating and Drying Defects

Cited by 81 publications

References 0 publications

Simultaneous multilayer coating of water-based and alcohol-based solutions

Simultaneous multilayer coating of water-based and alcohol-based solutions

Slot coating minimum film thickness in air and in rarefied helium

Operating windows of slot die coating: Comparison of theoretical predictions with experimental observations

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