DIP COATING APPLICATIONS
Dip coating is a useful technique when it is desirable to coat both sides of a part with the same coating. One advantage of the dip coating process is the simplicity over other coating techniques. Dip coating involves the deposition of a liquid film via the precise and controlled withdrawal of the substrate from a coating solution. It is low cost to set-up and maintain and can produce thin films w/ good uniformity. The ability to dip coat a part is sometimes limited by the size, shape and/or surface features of the part. For example, extremely large parts or parts that contain holes, protrusions, or other surface irregularities can be difficult to coat by the dip coating method. Dip coating is usually well suited for small sheets, vision, sport goggles, sunglass, and safety lens applications.
The basic equipment for a dip coating application includes a dip tank, where parts can be completely submerged into the coating, a closed loop circulation system, which circulates the coating from the tank through a filtering element, and a lift station mechanism to pull the part out of the coating tank at a controlled speed. Dip tanks are usually built using a “tank-in-tank” design. This design allows the coating that is pumped into the tank to overflow over the sides of the inner tank into the outer tank. The overflow, which is collected in the outer tank, is then pumped through a filtering element back into the inner tank.
Tank design is critical for the proper performance of the coated article. Tank size, the distribution of flow within the tank, and coating flow rate are all important parameters. Tank size defines the output of the system, whether a batch or continuous process is desired. The coating flow distribution within the tank should be uniform and free of mixing from the bottom to the top of the tank and across the entire cross section of the tank. The flow rate should allow for a sufficient volume turnover to properly carry debris out of the tank and into the filtering elements of the system. Generally, it is desirable to have a tank design and a circulation system that provide a high tank turnover rate and a smooth, turbulence-free coating solution surface. Similarly, it is desirable to have a part withdrawal mechanism that can accurately and smoothly control the rate of part withdrawal from the coating solution. A means for controlling the temperature of the coating solution is also a beneficial feature for a dip coating system. Controlling coating solution temperature; it is better to circulate a coating through a heat exchange device rather than trying to circulate the coating through a coil-type-chilling unit. A control temperature range of 15-25°C is suitable for most coating applications. Dip room atmospheric factors such as temperature, HEPA airflow, humidity, and part cleanliness all play an important role in film quality.
The circulation system should provide a sufficient turnover rate to adequately filter the coating solution. Ideally, the filtration system should consist of two or more filters in series, a configuration that allows for the use of a prefilter and finer micron subsequent filters. The optimum filtering configuration will depend on the coating being used and the rate of circulation through the filters. In general, the circulation rate should be set as high as possible without creating surface turbulence within the coating tank and without creating a large pressure differential (>10 psi) across the filter elements. A high-pressure differential across the filters can cause coating gels to extrude through the filter elements. The porosity and size of the filter elements can be adjusted to reduce a high-pressure differential, although this may also affect the cleanliness of the system. Dip coating draining in combination with solvent evaporation has to last long enough to ensure no surface defects such as blisters, pinholes or runs occur. To maintain a uniform coating, it is also necessary to replace the coating consumed by refilling, to maintain the right % of solids, solvents, and ph.
As in any coating application, the amount of coating that is deposited on the part, i.e., the coating thickness is a function of coating rheology; coating % solids, solvent composition, temperature, fluid density(ρ), viscosity (μ), and surface tension (σ). The coating thickness is also a function of the tank flow dynamics, proportional to the part withdrawal rate (U), and the angle of part withdrawal (a), as affected by gravitational acceleration (g). Normally, it is desirable to keep the coating properties constant and to use the withdrawal rate to control the coating thickness (h); a fast withdrawal rate will produce a thicker coating than a slow withdrawal rate. The Landau-Levich law for entrained film thickness describes such a relationship between the wet film thickness and the withdrawal speed of the substrate (when surface tension is considered):
Ca= μU/σ, h= 0.945Lc Ca2/3 where Lc is the capillary length Lc=√ σ/ ρg
The dip coating process involves a minimum of four unique steps: immersion, dwelling, withdrawal and drying. The critical steps for determining the properties of the deposited film are the withdrawal and drying stages. The final film thickness is determined during these stages by the interplay between the dragging forces, draining forces, and the drying of the film. Films are formed by the interplay of viscous flow, drainage, and capillary forces. By summing the contributions from the drainage and capillary formation, it is possible to obtain an equation that explains the thickness withdrawal speed relationship over a wide range of speeds. This also allows us to determine the minimum possible thickness that can be coated for a solution. Draining forces work to draw the liquid away from the substrate and back towards the bath. Conversely, entraining forces are those that work to retain fluid onto the substrate. The balance between these two sets of forces determines the thickness of the wet film coated onto the substrate. During the withdrawal stage, the formation of the wet film can be broken into four regions. These are the static and dynamic meniscus, the constant thickness zone, and the wetting zone. The static meniscus is determined by the balance of the hydrostatic and capillary pressures. The constant thickness zone is where the wet film has reached a given thickness. The dynamic meniscus and the flow of coating in this region determine the wet film thickness. Therefore, understanding the physics that underpins the curvature of the dynamic meniscus and the thickness of the stagnation point are important. The stagnation point occurs when the balance between the entraining forces and the draining forces are equal. It is the balance of these forces that determine final film thickness. For large sheets, there will be a dry film thickness variation between the bottom and top of the sheet
Good control over the coating thickness is important to achieve optimum performance of the PCI Labs’ VueGuard 900™ series of coatings. The coating thickness will affect the abrasion resistance and possibly other performance properties of a product. An optimized coating thickness can be determined empirically, according to the desired performance features of the final coated part.
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