Medium, or vehicle, selection is a critical decision in selecting the appropriate product for a glass decorating process. It is vital that the rheology and the medium system properties of the ceramic coating are appropriate for the final product and the application process. The rheological behavior of a coating can make the application process a success or failure.

Defining Rheology

Rheology, or viscosity, can be described as a measurement of the resistance to flow when exposed to a shear stress. Less resistance to flow results in lower viscosity; higher resistance to flow results in higher viscosity. When measuring rheology using a viscometer, it is important to factor in the general thickness/tackiness of the paste, as well as the typical shear rates it will experience during processing with a particular rheometer type, as well as spindle type/number.

Rotational viscometers can be used to test both Newtonian and non-Newtonians fluids and measure a wide range of viscosities. Three types of rotational viscometers are each used for different general viscosity ranges: LV series (low viscosity); RV series (medium viscosity), which is recommended for ceramic slurries, paints, inks, and coatings; and HA/HB series (high viscosity). Typically, a viscometer measures the torque required to rotate a spindle in a fluid. The torque required to turn the spindle is quantified as the shear stress acting across the surface area of the spindle. The fixed speed of the viscometer defines the shear rate. For a given viscosity, the resistance to flow, or torque, is proportional to the spindle’s speed of rotation and is related to the spindle’s size and shape. When measuring viscosity, a few key ideas are involved:

•  Viscosity range—Speed and torque ranges affect the viscosity measurement range of a rotational viscometer.

•  Spindles and speeds—Choosing the right spindle and speed is a trial-and-error process. Each spindle measures a different range of viscosity. Viscosity range is inversely proportional to the size of the spindle and to the rotational speed. It is commonly suggested to use a spindle and speed that will obtain a torque reading between 15-95%.

•  Sample size—Using too little or too much of a sample can affect the viscosity reading.

•  Constant temperature—Paste/liquid temperature can play a large role in the viscosity measured. They have an inverse relationship: as temperature increases, viscosity decreases.

•  Relaxation—After mixing/stirring/shaking, it is critical to give the paste/liquid time to relax and any residual shear stress to dissipate.

•  Run time—At lower rpms, it is necessary to measure for a longer period of time (~ 2-4 min) in order to allow the paste to equilibrate at that given speed. At high shear rates, less time is needed (~ 15-30 sec) because the paste does not require as much time to equilibrate.

Programs and temperature baths are helpful in controlling the measurement conditions and ensuring consistent testing.

Rheological Behaviors

A few general types of rheological behaviors are commonly associated with slurries/pastes, including pseudoplastic, thixotropic, Newtonian, and dilatant. Pseudoplastic materials display decreasing viscosity with an increasing shear rate (shear thinning behavior), while thixotropic materials can display decreasing viscosity with increasing shear rate, as well as decreasing viscosity with time at a constant shear rate. Newtonian liquids display a near constant viscosity with increasing shear rate. Lastly, dilatant materials display increasing viscosity with an increasing shear rate (shear thickening behavior); this behavior is not ideal for most application processes. See Figure 1 for examples of typical viscosity curves for various rheology types.

In practice, screen printing requires a shear-thinning viscosity behavior, and roll coating requires a less shear-thinning viscosity behavior, almost approaching a Newtonian liquid. The reason that screen printing requires a shear-thinning rheology is due to the shears created throughout the process. When the paste is resting on the screen (before/after printing) and the substrate after it is printed on the substrate, the shear rate is low. During these stages, it is important to have a higher viscosity. This ensures that the paste does not drip through the screen, and that the paste does not flow excessively out on the substrate, but rather retains the definition dictated by the screen pattern.

A key point, however, is that some flow-out of the paste after printing is important in order to eliminate any possible pinholes and help create a smooth surface. During the print stroke itself, which has a higher shear rate, it is important to have a low viscosity in order for the paste to flow through the mesh of the screen. The key concept is that the paste rheology needs to be controlled to work correctly with the print set-up and the desired properties of the final print.

In the roll coating process, it is typical to have a slightly shear thinning rheology for the paste, because the flow out of the paste on the substrate is critical to obtaining a uniform enamel thickness for the final product. Banding and stamping applications typically require a slightly tacky material with some shear-thinning behavior, while spray coatings usually use low-viscosity, shear-thinning materials. The key to understanding the rheology curve necessary for a process is understanding the shear characteristics of the process, as well as the desired flow, or lack thereof, required at the different stages of the application process.

Mediums Systems

Mediums for glass or ceramic coatings are blends of several materials created in order to achieve the desired performance properties. These materials typically consist of solvents, which are used to dissolve binder resins and provide the appropriate drying rate; binder resins, which are used to supply green strength, bond the enamel to the glass, and give certain viscosity characteristics to the medium and resulting paste; dispersants and wetting agents, which are used to wet and disperse ground glass and pigment particles; and rheology modifiers, which are used if necessary to provide special viscosity profiles. These materials affect the key properties of the pastes, as seen in Table 1.

A variety of different medium systems are available for the various application methods of the ceramic coatings. Conventional, solvent-based mediums require solvents to clean screens and equipment. Commonly used solvents include D-limonene, aromatic hydrocarbons (SC 150), xylene or mineral spirits. Conventional, solvent-based medium systems are typically used in applications such as screen printing automotive windshields, sidelites and backlites; screen printing appliance glass; screen printing conductive paste on automotive or flat glass; or screen printing or roll coating spandrel glass. They have also been used as a spray medium after dilution with solvent.

Water-miscible mediums can use water to clean screens and equipment. Note that local regulations must be observed for the disposal of waste water. Water-miscible medium systems share the same typical application process of the conventional, solvent-based medium systems.

Ultraviolet (UV) mediums are cured using ultraviolet light. UV medium systems are most often used for screen printing automotive windshields, sidelites and backlites, as well as for screen printing appliance glass.

Thermoplastic mediums use stearyl alcohol/paraffin wax as solvent components. This medium type results in solid materials that need to be heated to approximately 170°F to become fluid enough for screen printing. The thermoplastic nature of these systems eliminates the drying step and can be over-printed almost immediately. Typical applications for this type of medium system include screen printing bottles, tumblers and mugs. Some systems find limited use on small automotive sidelites. Mineral spirits and mineral seal oil are common cleaning solvents.

Banding mediums are usually applied with a banding wheel machine. The banding wheel picks up the enamel and transfers it to the ware being decorated. Typical cleaning solvents include D-limonene, aromatic hydrocarbons (SC-150) and mineral spirits. Banding medium systems are used for decorating bands onto dinner plates, cups and mugs. These systems can be used for precious metals, as well as glass and ceramic color decorations. The advantages and disadvantages of each of the various medium systems are shown in Table 2.

Alternative Options

Two specialty-use medium systems do not fit into any of the groups mentioned and are less commonly used: spray mediums and stamping mediums. Spray mediums are usually applied to coat hollow ware, bottles, tumblers and mugs, as well as some flat glass and architectural glass products. When using spray mediums, oxidizers may be necessary to coat the inside of globes or other hollow items in order to prevent ash entrapment during firing, and resulting discoloration. These mediums are generally water-based and can be cleaned up using water.

Stamping mediums are traditionally used for transferring trademarks, logos, or decorations onto dinnerware or flat glass. These mediums are solvent- or polybutene-based and can be cleaned up with solvents such as D-limonene and denatured alcohol.

Understanding the Process

In order to select the appropriate medium system, a number of factors need to be considered. First, an understanding of the rheology requirements of the process as it relates to the parts is needed in order to identify the appropriate rheology behavior and viscosity curve for the ceramic paste. Next, a thorough understanding of the processing of the part post decoration must be achieved: Will the part be fired wet or dry? Does it need green strength? If so, to what degree? What will the firing/curing profile of enamel paste be? What are the requirements for cleaning the paste off the equipment, and how does this impact the equipment and the employees?

It is important to ask and understand the answers to these questions in order to properly identify the appropriate medium system and avoid future issues during manufacturing. With a strong understanding of the process from start to finish, and a basic understanding of the various medium systems, getting the appropriate medium system for the job becomes a much easier task.  

 Editor’s Note: This article is based on a presentation given at the Deco ’14 meeting of the Society of Glass and Ceramic Decorated Products (SGCDpro). For more information, visit www.sgcd.org.