How it works

Dip Coating

Dip coating refers to the immersing of a substrate into a tank containing coating material, removing the piece from the tank, and allowing it to drain. The coated piece can then be dried by force-drying or baking. It is a popular way of creating thin film coated materials along with the spin coating procedure.

Stages of Dip Coating

The dip coating process can be, generally, separated into 3 stages:

  • Immersion: the substrate is immersed in the solution of the coating material at a constant speed preferably judder free

  • Dwell time: the substrate remains fully immersed and motionless to allow for the coating material to apply itself to the substrate

  • Withdrawal: the substrate is withdrawn, again at a constant speed to avoid any judders. The faster the substrate is withdrawn from the tank the thicker the coating material that will be applied to the board.

Pros and Cons

  • Owing to its simplicity, this method lends itself to automation. Film thickness is controlled by coating viscosity and rate of withdrawal from the tank. Dip tanks come in all shapes and are sized to accommodate the largest object to be coated.

  • Dip coating has its drawbacks, including: light parts tend to float and fall from the conveyor; film thickness can vary from top to bottom ("wedge effect"); fatty edges develop on the bottom of parts as excess coating drains; and refluxing by the solvent vapors above the tank removes some of the coating.

Brief Theory

With courtesy of H. Schmidt, M. Mennig

INM, Institut für Neue Materialien, Saarbrücken, Germany

Dip coating techniques can be described as a process where the substrate to be coated is immersed in a liquid and then withdrawn with a well-defined withdrawal speed under controlled temperature and atmospheric conditions. The coating thickness is mainly defined by the withdrawal speed, by the solid content and the viscosity of the liquid. If the withdrawal speed is chosen such that the shear rates keep the system in the Newtonian regime, the coating thickness can be calculated by the Landau-Levich equation [1] (eq. 1).

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Using: h = coating thickness, η = viscosity, γLV = liquid-vapour surface tension, ρ = density and g = gravity.

As shown by James and Strawbridge [2] for an acid catalyzed silicate sol, thicknesses obtained experimentally fit very well to calculated values. The interesting part of dip coating processes is that by choosing an appropriate viscosity the coating thickness can be varied with high precision from 20 nm up to 50 µm while maintaining high optical quality. The schematics of a dip coating process are shown in figure 1.

Fig. 1:Stages of the dip coating process: dipping of the substrate into the coating solution, wet layer formation by withdrawing the substrate and gelation of the layer by solvent evaporation

Fig. 1:Stages of the dip coating process: dipping of the substrate into the coating solution, wet layer formation by withdrawing the substrate and gelation of the layer by solvent evaporation

If reactive systems are chosen for coatings, as it is the case in sol-gel type of coatings using alkoxides or pre-hydrolyzed systems - the so-called sols - the control of the atmosphere is indispensable. The atmosphere controls the evaporation of the solvent and the subsequent destabilization of the sols by solvent evaporation, leads to a gelation process and the formation of a transparent film due to the small particle size in the sols (nm range) [3]. This is schematically shown in figure 2.

Fig. 2: Gelation process during dip coating process, obtained by evaporation of the solvent and subsequent destabilization of the sol (after Brinker et al [3])

Fig. 2: Gelation process during dip coating process, obtained by evaporation of the solvent and subsequent destabilization of the sol (after Brinker et al [3])

In general, sol particles are stabilized by surface charges, and the stabilization condition follows the Stern’s potential consideration [4]. According to Stern’s theory the gelation process can be explained by the approaching of the charged particle to distances below the repulsion potential. Then the repulsion is changed to an attraction leading to a very fast gelation. This takes place at the gelation point as indicated in figure 2. The resulting gel then has to be densified by thermal treatment, and the densification temperature is depending on the composition. But due to the fact that gel particles are extremely small, the system shows a large excess energy and in most cases a remarkably reduced densification temperature compared to bulk-systems is observed. However, it has to be taken into consideration that alkaline diffusion in conventional glasses like soda lime glasses starts at several hundred degrees centigrade and, as shown by Bange, alkaline ions diffuse into the coated layer during densification. In most cases, this is of no  disadvantage, since  the  adhesion  of  theses  layers becomes perfect, but influences on the refractive index have to be taken into consideration for the calculations for optical systems.

Dip coating processes are used for plate glass by Schott, based on developments of Schröder [5] and Dislich [6,7] for solar energy control systems (Calorex®) and anti-reflective coatings (Amiran®) on windows. The dip coating technique is also applied for optical coatings, e.g. on bulbs, for optical filters or dielectric mirrors by various SMEs and other companies, fabricating multilayer systems with up to 30 or 40 coatings with very high precision.

References:

  1. L. D. Landau, B. G. Levich, Acta Physiochim, U.R.S.S., 17 (1942) 42-54

  2. I. Strawbridge, P. F. James, J. Non-Cryst. Solids, 82 (1986) 366 - 372

  3. C. J. Brinker, A. J. Hurd, K. J. Ward in Ultrastructure Processing of Advanced Ceramics, eds. J. D. Mackenzie and D. R. 4Ulrich, Wiley, New York (1988) 223

  4. O. Stern Z. Elektrochem. (1924) 508

  5. H. Schröder, Physics of Thin Films, Academic Press, New York - London, vol. 5 (1969) 87 - 141

  6. H. Dislich, Angew. Chem. Int. Ed. 6 (1971) 363

 
 

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