Bar Coating: Methods, Theory and Applications

Bar coating is a simple wet processing technique used to deposit a thin layer of solution onto a substrate. It involves a bar (such as a Mayer rod) being placed above a substrate and dragged over a pool of fluid, resulting in the fluid being spread into a thin film or coating. Bar coating is commonly used in the development of applications such as automotive paints, photovoltaic cells and lithium ion batteries.

Bar coating can be performed manually or automated for greater precision and efficiency. In automated bar coating, a bar coater (such as a wire-wound bar) is moved across the substrate at a uniform speed using an automatic film applicator. The applicator moves the bar coater at a uniform speed, ensuring consistent and smooth film deposition. This coating method is also referred to as drawdown coating and rod coating, or wire bar coating and Mayer bar coating, depending on the type of bar used.

Bar Coater Design and Structure

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Bar coating is very similar to blade coating; solution is spread across a substrate by a cylindrical bar with wire spiralling around it. The gaps between the bar (or encasing wire) and the substrate control how much solution is allowed through. This determines the film thickness. The process can be optimized by altering:

• Bar height and pressure
• Bar speed
• Solution concentration
• Solution viscosity.

Manual bar coating suffers from low reproducibility and repeatability because parameters such as pressure and coating speed are not accurately controlled.

To produce consistent coatings, a bar coating machine can be used. These machines typically allow the use of different types of bar attachments, providing flexibility for various applications. When coating flexible substrates with wire bars, a firm rubber mat is sometimes placed beneath the substrate. This setup increases reproducibility by ensuring consistent pressure and gap, and by helping the fluids to self-level. Glass bases can also be used to give an even more uniform coating.

In some cases, a gravity-fed reservoir is attached to the bar. These reservoirs can help achieve uniformity in the coating. They also reduce waste at the beginning and end of the coating process. The reservoir removes controls the ink deposition, so can increase the reproducibility of coatings. These are particularly useful for long automated coaters where the required amount of ink cannot be deposited all at once. However, reservoirs are usually not needed for small-scale hand bar coating applications.

Types of Bar Used in Bar Coating

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The type of bar used in bar coating significantly influences the resulting coating, and some bars may not be compatible with all coating solutions.

Smooth, Open Wired or Closed Wired

A smooth bar, the simplest type, is a plain rod suspended a known distance above the substrate and pulled through the ink to leave a wet film. Over the width of the coating area, the rod does not come into contact with the substrate.

A wire bar (otherwise known as a Mayer bar or Mayer rod) come in two varieties: open wound (less common) and close wound (more common). These bars feature a rod wrapped with additional wire. This wires is what makes contact with the substrate while coating and so the pitch and diameter of the wire affects the coating's thickness.

In a close wound wire bar, the wire is wound as densely as possible, completely covering the rod. This configuration typically produces a thinner wet film and is suitable for less viscous fluids that can easily self-level.

On the other hand, open wound wire bars have wires spaced further apart, exposing some of the rod. The pitch of the winding becomes a critical variable, altering the wet film thickness. Open wound rods are generally better suited for more viscous fluids because the fluid needs to self level less once the rod has been drawn through it.

Bar Material

Stainless steel is the most commonly used material for both the rod and the wires due to its chemical resistance and high strength, making it less susceptible to damage. Stainless steel is readily available in different diameters and can be easily assembled into a wire bar. For particularly reactive or corrosive inks, different types of steel can be used to enhance resistance. While stainless steel is standard, bars can be made from any inert material that can be processed into the correct geometry.

Bar Coating Theory

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Coating Thickness

In wire bar coating, the wet coating thickness is determined by the size of the gaps between the wires. Larger diameter wires create larger gaps. They allow more coating material to pass between each wire, resulting in a thicker wet coating.

Self Leveling

Immediately after deposition, the wire bar coating leaves streaks and the film looks uneven. However, capillary forces then instantly attract the coating solution together to quickly form a continuous wet film. In reality, the relationship between wet coating thickness and wire diameter can be more complex. It can be affected by factors such as substrate absorbance, interactions between the wire bar and coating solution and coating solution viscosity - but these mainly affect the uniformity of the final film thickness. However, most manufacturers quote empirical relationships between a given coating bar and the wet coating thickness.

Bar Speed

Bar speed can also affect the final bar coating thickness. The faster the coating bar moves relative to the substrate, the thinner the coating. This is due to two main reasons. Firstly, the increased speed decreases the contact time between the coating and substrate during the coating process. You can imagine the bar coating process centering around moving the solution from the bar to the substrate. So, if the solution has a shorter contact time with the substrate, this transfer is hindered affecting the film.

Additionally, many coating solutions are prone to shear thinning, including paints and polymeric coating solutions. A higher coating speed applies more shear to the coating solution. This reduces the coating solution's viscosity, so it spreads more creating a thinner layer.

Minimum and Maximum Thickness

The minimum and maximum coating thickness depends significantly on the viscosity of the coating solution. For example, higher viscosity materials can be applied to give thicker coating thicknesses because they resist spreading. Whereas, lower viscosity materials can flow and spread out, limiting maximum thickness.

Dry vs. Wet Film Thickness

Theoretical dry coating thickness can be directly correlated to wet film thickness, if you know the the solid content of your coating solution.

This is based on conservation of mass arguments. No coating solution is lost during the process, the solvent evaporates leaving the solute layer only.

Advantages of Bar Coating

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Low Cost, High Simplicity

One advantage of bar coating is its low cost compared to other coating deposition methods. This is because bar coating does not require expensive pumps or precisely machined components. In fact, the simplest set-ups don’t even require a motor - the bar can simply be dragged by the operator. This hands-on approach is commonly termed “manual” bar coating. However, automated equipment can offer more precise control over the process and greater levels of repeatability.

Variable Coating Thicknesses

Bar coating can achieve a wide range of wet coating thicknesses from 10μm up to millimeters. It is quick and easy to change the coating thickness. Each bar gives a different wet coating thickness. Bar coating can be used to deposit very thick coatings up to several millimeters thick.

Flexible and Versatile

Additionally, bar coating is compatible with a wide range of solutions and substrates, enabling coating on both hard and flexible substrates.

Large Area Coating

With bar coating, you can create uniform thin films over sample areas ranging from several millimeters to meters in width. Commonly, bar coating is used to prepare samples in the A4 size range (210 x 297 mm / 8.3 x 11.7 in), which is useful for further laboratory testing. Bar coating can also be implemented onto roll-to-roll set-ups by coupling the bar coater with a pump which continuously feeds fluid onto the substrate before the bar.

Quick and Easy

Bar coating is also a quick process - the coating process itself usually takes less than a few seconds. Set-up and cleaning is also fast compared to other wet processing methods; there’s no complex calibration procedure and the wasted material is minimal. The wet and dry coating thickness can also be simply calculated, further increasing bar coating’s ease of use.

Bar Coating Limitations

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Disadvantages of bar coating include:

• Bar coating is susceptible to small differences in experimental procedure, so it can be difficult to get repeatable coatings.
• It can be difficult to coat substrates with an extremely thin coating.
• Wire-bar coating is also incompatible with very high viscosity coatings or coatings containing large, unstable particles. These prevent a continuous film from re-forming and “clog” the spaces between the wire, respectively, giving non-uniform coatings containing streaks

Bar Coating Applications

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The simplicity of bar coating means that it has been used extensively for all types of coatings. The applications range from small-scale research and development to large-scale industrial manufacturing, demonstrating its scalability and broad utility.

In Research and Academia

Academic literature contains lots of examples of bar coating at small scale. Bar coating is often used at the research and development stage of functional materials such as:

• Catalyst coatings
• Battery developments
• Organic semiconductors
• Perovskite solar cells

At these scales, bar coating allows new device architectures to be rapidly developed.

By using patterned substrates with differing wettability properties, you can produce targeted differences in both film composition and morphology across the width of your film. This also allows the vertical layering of these materials. Bar coating can equip you with a wide array of thin film fabrication choices.

Scalability is an important topic in research, as you want to make sure that results you see in the lab can be replicated in real life applications. Bar coating techniques are ideally suited for scalability experiments and for other continuous processes, such as roll-to-roll (R2R) manufacturing. Knowledge gained from bar coating is directly transferable to other industrial wet processing techniques such as gravure coating and slot die coating. R2R processes are crucial for large-scale production, where maintaining uniformity and precision across extensive substrate lengths is essential. This adaptability ensures that innovations developed at the laboratory scale can be efficiently translated into commercial manufacturing processes.

In Industry

In large-scale industrial applications, bar coating is used for high-volume production of various coatings and films.

• One of the primary industries that uses bar coating is the automotive sector. Automotive paints and protective coatings are deposited using bar coating techniques, ensuring a uniform finish that enhances durability and appearance.
• The packaging industry also extensively uses bar coating for applying barrier coatings to flexible packaging materials. These coatings improve the shelf life of packaged goods by providing moisture, oxygen, and chemical barriers.
• Bar coating is also used to produce adhesive labels and tapes, where consistent coating thickness is very important.
• In the electronics industry, bar coating aids the fabrication of printed circuit boards (PCBs) and flexible electronics. Conductive inks and dielectric materials are applied to create intricate electronic circuits, especially in the development of wearable electronics and flexible displays.
• Textile industries benefit from bar coating through the application of functional coatings to fabrics. Example coatings can include water repellency, flame resistance, and antimicrobial coatings.

Practical Tips for Bar Coating

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This is a useful checklist to help a user get the best films using bar coating techniques. Consider the following things before you attempt your first bar coating.

1. Ensure your substrate and bar coater are clean. Any dirt or debris can ruin the final coating. For example, dirt on the bar can result in defects throughout the final coating. If the substrate is dirty, this can affect the film from the point the dirt is picked up.
2. Excess ink at the start and the end of coating should be cleaned off so as not to disturb the rest of the coating. Consider using separate detachable pieces of substrate to catch excess solution at the start, end and sides of your coating material.
3. Make sure your substrate is held firmly in place against a clean, flat surface or a rubber bar coating mat.
4. You may need to use a surface pre-treatment or surfactant in order to achieve a thin film.
5. Use a spirit level to make sure your bar coating set-up is level in all directions.
6. You should carefully control the amount of ink deposited for a unit width of the coating area, ensuring that the ink is deposited evenly. A syringe, pipette or micropipette can be used for this.
7. Keep speed and pressure constant across different runs - if using manual bar coating, take a mental note of the speed and pressure used. Alternative, you could use an automatic film applicator to ensure that the bar moves at a consistent speed.
8. Curing conditions has a large influence on final morphology. Carefully consider what conditions are used for drying.
9. Carefully select the solvent used for cleaning the bar between coatings. This can impact the interactions between the bar and coating solution, which can influence the final coating thickness.
10. Use other techniques like ellipsometry and profilometry to confirm exact coating thicknesses. If exact film thickness is extremely important, don't just assume uniform film thickness.

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