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2022 | Buch

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Premetered Coating Methods

Attractiveness and Limitations

verfasst von: Peter M. Schweizer

Verlag: Springer International Publishing

Buchreihe : Engineering Materials

Enthalten in: Springer Professional "Wirtschaft+Technik" , Springer Professional "Technik"

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Über dieses Buch

This book compares premetered methods to self-metered processes and addresses general properties of premetered methods. It presents specific properties of slot, slide, and curtain coating. The book is divided in three parts:

Part I compares premetered methods to self-metered processes. It explains the term “premetered,” which is an expression of the law of mass conservation, and discusses the physical fluid properties that are relevant for premetered processes. Furthermore, it presents in detail the various basic flow fields that make up premetered coating methods. Lastly, it introduces the concepts of wall shear stress, residence time, and hydrodynamic assist to dynamic wetting.

Part II addresses general properties of premetered methods, such as the fluid conditioning and delivery systems, the nominal film thickness, and the film thickness uniformity, both in machine and cross-web directions (die design). It lists the attractive features of simultaneous multilayer applications, including an explanation of how mixing of adjacent layers can be prevented. This section concludes by revisiting examples of economic considerations.

Part III presents specific properties of slot, slide, and curtain coating. It examines various topics, such as coating equipment and coating configurations, coating modes, details of the various flow fields, operating window and process limitations, and process optimization.

Inhaltsverzeichnis

Frontmatter

R&D in Premetered Coating Technology

Frontmatter

1. Introduction

Abstract

The introduction to this book re-captures the history of the R&D work in coating technology carried out and disseminated across the world. It prepares a platform for this book, which adds another piece to the vast puzzle of information about coating technologies.

Peter M. Schweizer

2. Premetered Versus Selfmetered Coating Methods

Abstract

The class of premetered coating methods including slot, slide, and curtain coating is compared to the class of selfmetered methods, which comprises some 50 variations of roll and blade coating processes. Process limitations and application ranges for relevant operating parameters are listed, and the ease of implementing process-enhancing concepts in terms of product uniformity and process productivity are discussed. It is concluded that premetered coating methods are preferred over selfmetered processes.

Peter M. Schweizer

3. Mass Balance and Flow Rates

Abstract

The physical law of mass conservation is basic for premetered coating methods. This chapter summarizes correlations between various thickness measures of the coated film and related flow rates that are required for obtaining coated films with a desired thickness. Specifically, the wet thickness of the coated film, which is the relevant film dimension from a coating and fluid mechanics point of view, as well as relevant flow rates are expressed in terms of the dry thickness of the coated film, which is the relevant dimension from a product performance point of view. Alternatively, thickness and flow rate parameters are expressed in terms of the dry coat weight, which is more widely used in the converting industry.

Peter M. Schweizer

4. Physical Fluid Properties

Abstract

Coating involves fluid mechanics, and understanding coating requires understanding the various basic flow fields that may exist in a given coating method. The performance of a given flow depends on three classes of parameters, namely operating conditions such as flow rate, physical fluid properties such as viscosity, and geometric dimensions such as pipe diameter. This chapter discusses physical fluid properties that are relevant for coating flows, including density, viscosity, surface tension, contact angle, wettability of substrates, as well as their dependence upon the solids concentration and temperature. Regarding the viscosity, equations for modeling the dependence upon the shear rate are presented. Finally, methods and techniques for measuring the fluid properties are reviewed.

Peter M. Schweizer

5. Basic Flows of Premetered Coating Methods

Abstract

Premetered coating methods comprise various basic flow fields including pipe, duct, slot, film, curtain, and boundary layer flow, as well as creeping flows that occur in the coated film before solidification. This chapter reviews theoretical models for analyzing and optimizing these flows. The focus is on analytical models because they are very well suited for visualizing the underlying physical concepts that control the flows. Also important is the use of the local power law model proposed by Ruschak and Weinstein (Polym Eng Sci 2301–2309, 2014), which, in combination with analytical flow equations and modest computational efforts, allows an accurate modeling of a complex behavior of the shear viscosity as described by the Carreau-Yasuda equation. In this context, various strategies for solving flow problems are proposed. Finally, for each basic flow field, potential locations for flow separation are visualized and ways for preventing the onset of vortices are suggested.

Peter M. Schweizer

6. Wall Shear Stress

Abstract

This chapter discusses wall shear stress, which is a fluid mechanical parameter that depends on the viscosity, the flow rate, and the geometry of the flow system. Wall shear stress is also a measure of the self-cleaning capability of a flow system and can increase the productivity of a coating process by reducing the non-productive time related to cleaning and product change-over-time. This chapter introduces time-dependent parameters such as the mean residence time, the residence time spectrum, the transfer function, and the change-over-time. The chapter illustrates how these parameters are controlled by wall shear stress, and so, how the residence time characteristics of a given flow system can be optimized by smart equipment design. Results can be applied to confined flows such as the fluid delivery system and the slot die. Controlling the residence time behavior of a coating process is essential for handling chemically reactive fluids.

Peter M. Schweizer

7. Dynamic Wetting and Hydrodynamic Assist

Abstract

The concept of the hydrodynamic assist in dynamic wetting was first presented by Blake, Clarke and Ruschak from Kodak. Hydrodynamic assist is a property common to all premetered coating methods. Specifically, the momentum of the approaching liquid is exploited to postpone the onset of air entrainment to higher web speeds by placing the location of the dynamic wetting line into the projection of the approaching liquid stream. This chapter discusses why premetered coating processes are superior to self-metered methods with regard to dynamic wetting. It also explains that exploring hydrodynamic assist requires process optimization to ensure that the dynamic wetting line is indeed located inside the impinging liquid stream and not too far upstream or downstream.

Peter M. Schweizer

General Properties of Premetered Coating Methods

Frontmatter

8. Preparing, Conditioning, and Delivering Coating Fluids

Abstract

Elaborate fluid conditioning and delivery systems are an integral part of all premetered coating methods. Achieving a high uniformity of the coated film is impossible if the fluid is not uniform and homogeneous in terms of the solids concentration and temperature. This chapter illuminates the various processes and the associated equipment that might be included in a well-designed fluid conditioning and delivery system. Specifically, a layout of an ideal delivery system is proposed. Also, properties and requirements of the following processes and equipment are discussed: degassing, pumping, measuring flow rate, filtering, controlling temperature, mixing, changing coating fluids, damping pulsations, recycling fluids, cleaning-in-place, designing vessels and pipe-line systems.

Peter M. Schweizer

9. Film Thickness and Film Thickness Uniformity

Abstract

One of the attractive features of premetered coating methods is the fact that the nominal thickness of the coated film is pre-determined by the law of mass conservation. For a constant coating width, in particular, the thickness only depends on the flow rate/width and the coating speed. Interesting is the question about the uniformity of the nominal film thickness, both in the machine and cross-web direction. This chapter shows that the film thickness uniformity in the machine direction is controlled by the design of the pump as well as the uniformity of the motors that drive the pump and pull the substrate through the coating machine. Achieving a good film thickness uniformity in the cross-web direction is more complicated because it depends on the design and performance of the slot die. Therefore, a large portion of this chapter is devoted to die design. Specifically, theoretical models for designing the die-internal geometry are presented, the performance of such dies is analyzed, and ways for optimizing die design are proposed.

Peter M. Schweizer

10. Concepts for Varying the Coating Width

Abstract

The choice between variable or constant coating width and the choice between variable or constant web width bears significant consequences for the product quality and process productivity, and hence to the manufacturing costs of coated products on the one hand, and with regard to the investment costs for coating equipment on the other hand. The process of selecting the best number and size of different coating and web widths in an attempt to minimize both the investment and operating costs is a classic optimization problem. In this chapter, various positive and negative aspects of the options for changing the width are considered and evaluated.

Peter M. Schweizer

11. Splice Passage and Coatability

Abstract

Slot and slide coating are characterized by narrow gaps between the die lip and the substrate surface that extend across the full coating width. In contrast, curtain coating does not have narrow gaps across the width, but they exist between the bottom of the curtain edge guides and the substrate at the edges of the coated film, and upstream of the impinging curtain between the suction baffle and the substrate. These gaps must be designed and operated in such a way that the splice between subsequent substrate rolls can pass through the gaps without hindrance. This chapter describes the type of splices that are typically encountered in the converting industry. In addition, operating instructions are proposed that minimize the harm inflicted by the splice to the uniformity of the coated film during the splice passage.

Peter M. Schweizer

12. Simultaneous Multilayer Coating Capability

Abstract

The ability of simultaneously applying several different liquid layers is an attractive feature of premetered coating methods. This chapter describes several ideas that improve the functionality of the coated product and/or increase the productivity of the coating process by designing multi-layered films.

Peter M. Schweizer

13. Mixing of Adjacent Layers

Abstract

This chapter addresses the question of interlayer mixing, be it by convection or diffusion. Convective mixing can be prevented if the multilayer film flows are kept laminar. Mixing by diffusion cannot be prevented, but ways are suggested to minimize adverse effects in the coated film.

Peter M. Schweizer

14. Guidelines for Designing Single-Layer and Multilayer Films

Abstract

Applying simultaneous multilayer coating technology, particularly slide and curtain coating, requires well-designed multilayer films that flow down on the inclined surface of the slide die and, in the case of curtain coating, through the curtain prior to being coated. These film and curtain flows must be optimized in terms of film thickness uniformity and fluid homogeneity because any flaws in these films will be transported through the coating process, and they will end up in the coated film where they deteriorate the uniformity of the final product. This chapter provides guidelines for designing singlelayer and multilayer liquid films.

Peter M. Schweizer

15. Modeling Economic Aspects of Coating Methods

Abstract

Economic aspects of coating processes, i.e., the effect of an installed coating method on the process productivity and the resulting product unit costs, can be modeled easily and accurately. Many details of such models were presented previously by Schweizer (1997).

Peter M. Schweizer

Specific Properties of Premetered Coating Methods

Frontmatter

16. The Concept of the Coating Window

Abstract

Coating windows are an instructive way to visualize various operating boundaries of a given coating process. In this chapter, suitable operating windows for premetered coating methods are presented and discussed. It is important to understand how the operating boundaries of the coating window can be moved such that a desired operating point comes to lie inside the window. Equally important is finding the optimum operating point inside the window that results in the best possible product uniformity and process productivity.

Peter M. Schweizer

17. Specific Properties of Slot Coating

Abstract

Slot coating is currently a popular application process, not the least for its capability to coat intermittently, which is a necessary feature for manufacturing electronic devices such as OPV’s and OLED’s, as well as electrodes for Li-ion batteries. After presenting possible process configurations and related equipment, the discussion turns to the operating window with the low flow limit as the major operating boundary. The onset of ribbing lines, air entrainment and vortices are other operating limits, and actions to prevent these flow defects are proposed. Then, the effect of the uniformity of the coating gap on the uniformity of the coated film, various operational aspects, and means to optimize the process are discussed. In the second part of the chapter, unique and attractive features of slot coating are presented, including tensioned-web coating, double-sided coating, stripe coating, intermittent, and pattern coating.

Peter M. Schweizer

18. Specific Properties of Slide Coating

Abstract

Slide coating was invented in the photographic industry by Eastman Kodak. Owing to its capability to apply several layers at the same time, slide coating resulted in a large productivity increase when compared to slot coating. The chapter begins with a description of the process configuration and the associated equipment, followed by a discussion of the flow field of the coating bead. The main operating limits of slide coating are the onset of ribbing lines and several vortices. The onset of ribbing lines is documented with experimental data. The chapter closes with suggestions for optimizing the process.

Peter M. Schweizer

19. Specific Properties of Curtain Coating

Abstract

This chapter starts out by listing all process-technological advantages that curtain coating has over any other application methods. The discussion concerning process configuration and equipment addresses issues such as the web path through and the design of the coating station, whether a slot or slide die should be used, the suction baffle for removing the air boundary layer on the uncoated substrate, the inboard and overboard operating modes, and how to start and stop the coating process. The coating window including the dominant operating limits is presented for both long and short curtains. Finally, guidelines for optimizing the coating process are proposed.

Peter M. Schweizer

Backmatter

Titel: Premetered Coating Methods
verfasst von: Peter M. Schweizer
Verlag: Springer International Publishing
Electronic ISBN: 978-3-031-04180-8
Print ISBN: 978-3-031-04179-2
DOI: https://doi.org/10.1007/978-3-031-04180-8

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