Dip Coating and Knife Edge Coating

Transcription

1 Dip Coating and Knife Edge Coating Y O G E S H K U M A R ( M A S T E R ' S I N C O M P U T A T I O N A L E N G I N E E R I N G ) F R I E D R I C H - A L E X A N D E R - U N I V E R S I T Ä T E R L A N G E N - N Ü R N B E R G G E R M A N Y 9 TH I N D O - G E R M A N W I N T E R A C A D E M Y , P U N E, I N D I A, D E C E M B E R ,

2 Agenda Introduction Self-metering and pre-metering coating Pre-metering coating Slot Coating Theoretical consideration of Dip coating Similarity consideration for Dip Coating Comparison with Literature Numerical Investigation and Computation Results Theoretical consideration of Knife coating Applications and Dosing Views of the pressure distribution on the knife-edge Basic geometry of the knife-edge coating Calculation of Pressure distribution Operating point determination of knife-edge The Analytical treatment of the knife-edge flow Amounts for layer thickness Extension to high speed knife-edge coating Conclusion References 2 December 17, 2010

3 Self-metering v/s Pre-metering Self-metering Pre-metering All the liquid fed into the coating die by a metering device: ex. Slot coating. Doesn t depend on fluid parameters. Coated layer thickness depends on fluid parameters. e.g For Dip coating U w1 = h 1 U w2 > U w1 h 2 > h 1 December 17,

4 Pre-metering coating technique Slot coating. Self-metering devices show film thickness variations. The film thickness is entirely controlled by the mass flow: Dip Coating and Knife Edge Coating Keeping the mass flow rate constant, the film thickness maintained at constant value. High precision coating can be obtained. Contactless coating permits high quality layers. December 17,

5 Simple and Inexpensive. Immersing a substrate in a vessel filled with liquid (fig a). Withdrawal of the substrate (fig b). Liquid in the vessel drained around the substrate (fig c). Continuous dip coating as a batch process (fig d). Most famous for coating irregular shaped and rigid body objects. Dip Coating and Knife Edge Coating Theoretical consideration of Dip Coating a + d b c (a) substrate immersion. (b) Coating by substrate withdraw. (c) Coating by drainage. (d) Continuous dip coating. December 17,

6 Dip Coating Substrate is withdraw vertically. Liquid next to the stagnation line ends up in the final film, whereas the liquid on the other side returned to the bath by gravity. Stagnation line location is linked to the thickness of the deposited film. Coating speed and angle of substrate withdrawal affect the meniscus shape and hence the position of the stagnation line. Film-Entrainment Region Static-Meniscus Region Stagnation Line Dip coating depends on several dimensionless parameters such as: Capillary no. (Ca), Bond no. (Bo), Dip no. (Di) December 17,

7 Similarity consideration for Dip Coating Film-Entrainment Region Static-Meniscus Region Stagnation Line All the parameter that have an influence on thickness H One Dimensional analysis Thickness of the dipping method depends on five parameters. Formation of curved surface. Substrate is pullout vertically from the bath tub. December 17,

8 Similarity consideration for Dip Coating 1. α+γ = 0 2. β-3α+1= β-2γ=0 Bo = Bond Number Setting up the equation for determining the unknown quantities. Solve the equations. Setting the α,β,γ and determine the formula. December 17,

9 Similarity consideration for Dip Coating Reduction from six to three parameter in dimensionless form. Determination of two characteristic velocities and length. Dimensionless film thickness depends on two parameters. December 17,

10 Comparison with Literature The case of dip coating has been extremely studied Landau + Levich (1942) C 1 = for Newtonian fluid Valid for low capillary numbers Ca 10-2 No dependency on 3 The analytical results can be verified experimentally. Good agreement is obtained for low Ca numbers. December 17,

11 Comparison with Literature Validation of numerical calculation. Thickness increases with increasing substrate speed. For low Ca numbers For high Re number December 17,

12 Numerical Investigation For the treatment of numeric's, the program used is Polyflow. Time-dependent equations. Choice of realistic BC s. Result provide details into the coating process. December 17,

13 Numerical Computation Correct dimension of the code. Structured computation grids. Validation by numerical calculations. Back flow causes flow circulation. Location of the flow inlet has no effect on the layer thickness. December 17,

14 Experimental setup USM Liquid bath 2. Uni-axial line unit 3. Ultrasonic weight measurement device (g/m 2 ) 4. Substrate December 17,

15 Experimental result W = g/m 2 Time (t) December 17,

16 Results Fluid with production speed is disadvantage. Increase of coating layer thickness with the capillary number Ca Gravity plays an important role. Too thick coating layer need to be partially scrapped off (unavoidable). December 17,

17 Knife edge coating Knife edge coating moves the substrate which is already covered with a coating liquid towards a knife. This knife removes all spare liquid, leaving only the desired layer behind. There are various process parameters to optimize, since the flows at the knife can become very complex. Numerically the flow can be studied. The Knife-edge coating is also widely used. December 17,

18 Knife edge coating - Application Large evaporation surface Limitation at high speed. No film-splitting. Low pressure area. Dosing method. Risk of deposits in applicator December 17,

19 Knife edge coating - Application Order and dosing in a system. Low penetration due to short residence. Advantage of compactness and lower cleaning costs. Improved operating characteristics at low application rates. Inverted knife-edge coating system. Required quantity adjustable via rotation speed of the roll applicator. Good surface quality, flexibility. At high speeds prone to ribbing, spraying December 17,

20 Knife-Edge dosing. Velocity vector in front of Knife-edge. Dimensionless ratios: Re = (rud)/µ = 85.6 Fr = U 2 /(gd) = Ca = (µu)/s = 5.08 h = H/D = 0.2 The Characteristics quantities are used to determine general solution. December 17,

21 Knife-Edge dosing. Pressure field Compressive force on knife side 48.8 N Compressive force on knife circulation 39.6 N Shear force on knife circulation 8.0 N Total force 88.2 N (perpendicular to the knife side) Pressure distribution on the substrate December 17,

22 Pressure distribution on the Knife-edge Flow behind the knife l R Coated film + Couette flow U w Channel flow (Pressure driven force) Pressure Velocities December 17,

23 Basic geometry for analytical view of the knife-edge coating This is the basic geometry of each knife-edge For the low-speed coating, without applying pressure, then: m/min is approximately the area of the low speed range for the bar coating. December 17,

24 Calculation of Pressure distribution P H l R H P 2 d R P 0 h U w Pressure difference at the back of knife: For plane channel flow: Ruschak relation: December 17,

25 Calculation of Pressure distribution The low-speed knife-edge coating can be theoretically treated very well. One can see from the parameters, which settings must be made to get a good knife coating. From the individual pressure distributions calculated by: and also: December 17,

26 Operating point determination of knife-edge For P H = 0 gives: Dip Coating and Knife Edge Coating Using the above equation can see how one can get to higher deposition rates. Increase in the coating speed is achieved for constant h. It is possible to write programs that can determine the coating point due to the increase in speed by increasing the fluid level. December 17,

27 The Analytical treatment of the knifeedge flow Knife-edge flow is derived from Navier-Stokes equations, With the below BC s the flow can be treated under knife-edge. Couette flow: Channel flow: Couette flow Channel flow Total flow December 17,

28 Amounts for layer thickness So volume flow per unit width In order to calculate the film thickness h: Contribution to Couette flow Coefficient of pressure gradient for channel flow Pressure gradient for channel flow Knife coating easy to understand physically. December 17,

29 Extension to high speed knife-edge coating For the high-speed knife-edge coating: Furthermore give: And also This results in following final equation: This equation can be solved by using programs such as "Mathematica", "Maple", Octave" etc.. December 17,

30 Summary & Conclusion Dip Coating Film thickness varies according to Landau and Levich relation at low values of capillary numbers e.g. Ca Simple but not suitable for high speed coating For higher value of Ca, its follows a power law, h + = 0.7 Ca 1/2 Knife-edge coating Suitable for high speed coating. Complex but desired thickness can be obtained. Coating itself is everywhere in our daily life. The optimized techniques require very elaborated computations and sophisticated engineering for the real applications. The field offers many possibilities for applied and scientific in the near future. December 17,

31 References Landau, L.D. and Levich, V.G., Dragging of a liquid by a moving plate, Acta, Phys. Chem., URSS, Vol. 17, pp , Thesis Report: Theoretical Treatments of Dip Coating Film Thickness Dependence on Fluid and Process Parameters by D. Schmidt, R.Mohanty, G.Zheng, A.Sengupta & F. Durst. Thesis Report: Theoretical treatment of Knife-edge coating window and its practical use by D. Schmidt, R. Mohanti, and F. Durst. Liquid film coating, scientific principles and their technological implications, edited by Stephan F. Kistler and Peter M. Schweizer. December 17,

32 Thanks for you kind attention December 17,