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Editor’s Note: The following paper won the 2013 John Matteucci Technical Excellence Award for Web Coating at the AIMCAL Web Coating & Handling Conference in Charleston, SC. An expanded abstract and PDF of the full paper is available for download on our Website:
Abstract
Slot coating is one of the preferred, precision coating methods in the manufacturing of single- and two-layer coated products. The thickness of the coated liquid layer, in principle, is t by the flow rate fed to the die and the speed of the substrate moving past, and is independent of other process variables, being ideal for high-precision coating. The region in the space of operating parameters where the delivered liquid layer is adequately uniform is referred to as a coating window . The coating window can be determined by extensive pilot-plant experiments or by a complete analysis of the coating flow and its stability limits. A summary of the fundamentals of the slot-coating process and its operability limits is discusd, together with some aspects of advanced rearch being pursued.
Introduction
I
n earlier days, coating technology was developed as an art. However, the coating process is a complex, multidisciplinary science that involves wetting, adhesion, fluid mechanics, rheology, chemistry, interfacial science and heat, and mass transfer. Competitive pressure reduces the time available to bring new products into the market, and process development through extensive pilot-plant trials may delay production. Thus, it is important to analyze the physical mechanisms responsible for the success or failure of manufacturing process. Process engineers should not pursue only process know-how, but also process know-why .
Fundamental understanding of basic mechanisms involved in all phas of the manufacturing of coated films, including liquid preparation, coating and solidification requires long-term investment in process rearch and development, and specially-designed experimental and numerical analysis tools.xuetr
Fundamentals of
the slot-coating process
By Professor Marcio Carvalho, Dept. of Mechanical Engineering, Pontifica Universidade Catolica do Rio de Janeiro (PUC-Rio)
Slot coating process has been deeply studied and understood. Here, we will discuss some of the fundamental aspects of this process and recent developments related to coating flow of particulate suspensions.
Slot-coating operability window
In slot coating, the liquid is
pumped to a coating die in which an elongated chamber distributes it across the width of a narrow slot, through which the flow-rate-per-unit width at the slot exit is made uniform. Exiting the slot, the liquid fills – wholly or partially – the gap between the adjacent die lips while the substrate translates rapidly past them. The liquid in the gap – bound upstream and downstream by gas-liquid interfaces, or menisci – forms the coating bead (e
Figure 1). The competition among viscous, capillary and pressure forces – and in some cas inertial and elastic forces – ts the range of operating parameters in which the viscous-free surface
flow of the liquid can be two-dimensional and steady, which is the desired state. To sustain the coating bead at higher substrate speeds, the gas pressure of the upstream meniscus is made lower than ambient, i.e.: a slight vacuum is applied to the upstream meniscus (Beguin, 1954).
Slot coating belongs to a class of coating methods known as pre-metered coating: the thickness of the coated liquid layer is t by
the flow rate fed to the coating
例假来了怎么办FIGURE 1. Sketch of the slot-coating bead
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continued on page 58 u
die, and the speed of the moving substrate and is independent of other process variables. Thus, pre-metered methods are ideal for high-precision coating. However, the nature of the flow in the coating bead and, therefore, the uniformity of the liquid layer it delivers can be affected by the substrate speed, the viscosity and any non-Newtonian properties of
the liquid, as well as the configuration of the die lips immediately upstream and downstream of the slot exit. The region in the space of operating parameters of a coating process – where the delivered liquid layer is adequately uniform – is usually referred to as a coating window. Refined flow visualization through a glass backup roll and finite-element modeling of the flow were ud to
analyze the limits of operability and flow stability within tho limits by Carvalho and Kheshgi (2000) and Romero,
Scriven and Carvalho (2004; 2006). The failure modes (shown in Figure 2) show that the coating window is bounded by three modes of failure:
High-vacuum limit: When the coated layer is thicker than the thinnest that can be produced at a fixed gap and by substrate speed (i.e.: t > t min in Figure 2), too great a vacuum at the upstream
FIGURE 2. Sketch of the slot-coating process window as a function of coating thickness, gap and vacuum pressure
t continued from page 57
MA TTEUCCI A W ARDS
free surface caus liquid to be drawn along the die
surface into the vacuum chamber. This diversion of
liquid destroys pre-metering.
Low-vacuum limit: Too little vacuum at the upstream
free surface leaves the net viscous drag force on the
upstream part of the bead unbalanced by the pressure
gradient that is impod by capillary-pressure forces
in the menisci upstream and downstream and the
difference in external pressure on tho menisci (i.e.: vacuum). As a respon, the upstream meniscus
shifts toward the feed slot until the bead drastically
rearranges into a three-dimensional form that
安妮日记delivers parate rivulets to the substrate. Between
the rivulets are dry lanes that extend upstream
回也不改其乐
风雨花through the bead. Along tho lanes, air is sucked
into the vacuum chamber. It is in this regime that – at
given vacuum (ambient pressure downstream minus
air pressure exerted on upstream meniscus) – there is
a lower limit to the thickness of a continuous, liquid
layer that can be coated from a downstream gap of
specified clearance. As Figure 2 shows, the limit can
be lowered by applying greater vacuum and, thereby,
成都特产有哪些shifting the upstream meniscus away from the edge
of the feed slot.
Low-flow limit: At a given substrate speed, too
low a flow-rate-per-unit width from the slot caus the downstream meniscus to curve so much that it cannot bridge the gap’s clearance. Conquently, the meniscus becomes progressively three-dimensional, alternate parts of it invading the gap until the bead takes a form that delivers parate rivulets or chains of droplets to the substrate moving past. This transition from a continuous, coated liquid layer is called the low-flow limit: the minimum thickness of liquid that can be deposited from a gap of specified clearance at a given substrate speed. As Figure 2 makes clear, it is independent of the vacuum applied, given that the vacuum is great enough to draw the upstream meniscus away from the feed slot.
御姐壁纸By understanding the physical mechanisms related to the low-flow limit, Carvalho and Kheshgi (2000)
were able to propo a way to delay the ont of this process failure, enabling the slot-coating of thin films at high speeds. Liquid inertia was ud to push the downstream meniscus such that it does not invade the coating bead, delaying the breakup into rivulets. Pilot-plant data together with the finite-element simulation in the high-speed regime is shown in Figure 3.
Slot-coating of particulate suspensions
The analysis prented considered the liquid as a Newtonian fluid. However, the liquids coated in practice are polymer solutions, particle suspensions or a combination of both. The complex flow in a coating bead may create a non-uniform particle distribution in the flow, leading to strong viscosity changes within the coating bead, which may affect process limits. Moreover, the flow may have a strong influence on the final particle distribution in the coated liquid that may be related directly to the microstructure and final product performance.
Recent developments from our group have combined the solution of fluid-flow equations that considers the liquid properties as a function of the local particle concentration coupled with a particle-transport equation to study how the process parameters affect the particle distribution in the coated film. Figure 4 shows how the flow affects the particle concentration on the coated layer. Throu
gh the feed slot, the particles move from the high-shear toward the low-shear region of the flow, leading to high particle concentration in the middle of the feed slot.
When the coating thickness is equal to half of the gap, the pressure gradient under the downstream die lip is negligible and the particle transport is weak, leading to a high particle concentration layer in the middle of the coated film. At lower values of the wet thickness, an adver pressure gradient is created under the die lip. At a thickness clo to one-third of the coating gap, the region of zero shear rate is located near the die lip. Particles move toward that area, leading to a high concentration near the die-lip surface and, conquently, on top of the coated film.
Conclusion
The examples discusd here show how fundamental understanding of coating flows and the physical mechanisms associated with different failure modes lead to better-designed process and can drastically reduce the
process development and production scaleup time required to bring new FIGURE 3. Ont of the low-flow limit at different conditions. The blue shaded area reprents the augmented process window associated with inertial effects.
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FIGURE 4. Particle concentration under the downstream die lip and near the downstream meniscus for two different coating thickness products to market. Fundamental understanding of coating
process is not easy. It requires collaboration between experts
from different disciplines; it is a result of a continuous effort and, therefore, requires time, investment and commitment. n
Marcio Carvalho, a professor in the Dept. of Mechanical Engineering, Pontifica Universidade Catolica do Rio de Janeiro (Brazil), holds a Ph.D. in Chemical Engineering from the University of Minnesota. He has worked as nior process development engineer at 3M Co. and Imation Corp. in the areas of pre-metered coating, roll coating and drying technologies. Marcio also is a member of th
e Graduate Faculty in the Dept. of Chemical Engineering &
Materials Science at the University of Minnesota. His rearch is
focud on veral aspects of coating
process, non-Newtonian fluid mechanics in small-scale flows, asymptotic methods and flow of emulsions in porous media. In 2004, he received the Young Investigator Award from the Intl.
Society for Coating Science and Technology (ISCST). Marcio can be reached at +55-21-3527-1174, email: msc@puc-rio.br; puc-rio.br/lmmp/
