Many industrial and protective coating materials consist of more than one component. A multi-component coating is any product that consists of two or more separate parts that are mixed together at a specified ratio. As the materials are mixed, a reaction occurs that allows the disparate parts to chemically cross-link and form a polymer. The most basic multi-component coatings consist of a base material and an activator. The addition of the activator to the base is what begins the chemical reaction. Some coating materials consist of more than two components and special attention must be given to the manufacturer’s directions regarding the order that the parts must be mixed. For example, the activator may need to be added last, once the other two components are already mixed so that the pot life isn’t abbreviated.
So, why is proper mixing of multi-component coatings so important? The component materials are designed by the coating manufacturer to react in a particular way when mixed with their corresponding parts. If the materials are not properly mixed, the chemical reaction will not occur as intended. This can result in reduced coating performance or even complete failure of the coating material. In some cases, there is visual evidence of the coating material not being properly catalyzed. An applied coating may sag or appear to be wet and uncured, or remain soft due to an incorrect proportion of activator to base material. In other circumstances, the coating material appears to be properly cured and exhibits no visual evidence of improper catalysis. However, the physical properties of the coating may be altered that can affect performance. A material may become brittle or be more susceptible to environmental weathering. If an improperly mixed coating is applied as part of a multi-coat system, the performance of other layers composed of properly mixed coating materials may also be affected.
To reduce the chance of coating failure from improper mixing, one must pay close attention to the tools employed and the instructions provided by the coating manufacturer. To facilitate uniform distribution of all components, mixing of multi-component systems should be performed using a power mixer (Figure 1). Other methods (paint shakers, flat stirring sticks, etc.) lack the capacity to ensure that the materials have been uniformly mixed. A power mixer consists of an air-driven drill and a mechanical mixing blade. Mixing blades are available in a range of sizes, so it is necessary to determine what size is required (based on the size of the kit) prior to mixing to achieve the proper results.
Once the correct mixing tool has been selected and prior to mixing, thoroughly read the manufacturer’s Product Data Sheet (PDS) that was provided with the coating material. Critical direction is provided in the PDS regarding mixing instructions, mix ratios, induction times (if required), pot life, etc.; safe use and personal protective equipment is listed on the Safety Data Sheet (SDS) for each component. After reading the PDS, examine the individual components for signs of degradation, gelation, or any visual evidence that a component has expired or been compromised and is not suitable for use. Once you have verified that the material is in good condition, begin by mixing the individual components using the mechanical mixing blade. Depending on the flash point temperature (listed on the PDS and SDS), flammable solvent vapors may be emitted from the cans in this process, so spark hazards (e.g., use of electric drills) and smoking should be prohibited. It is necessary to mix the individual components to verify that the materials have achieved a uniform suspension prior to combining them. Be sure to mix the coating in all areas of the container to prevent clumping or settling. It may be necessary to scrape the sides and bottom of the containers with a paint stick to confirm that the material is homogenous.
Once the components have reached a uniform consistency, they can be combined at the mix ratio indicated in the PDS (Figure 2). Coating components are pre-packaged by the coating manufacturer in the proper ratio, so it is recommended to mix complete kits when feasible. While 1:1 mix ratios are not difficult to portion out, a ratio of 3.5:1 is more challenging, and some products are portioned by weight, or a combination of weight and volume, which requires
graduated containers and scales. After combining the materials, additional mixing is required for the separate components to be uniformly blended so that the chemical reaction can occur. Some coatings require an induction time (sweat-in, digestion, and cook time are synonyms) to achieve a proper reaction. An induction is a period of time (e.g., 30 minutes) that the combined coating material is allowed to “simmer” (covered to prevent solvent loss) and the chemical reaction is able to proceed undisturbed. The required induction time will be indicated on the PDS and may vary depending on the ambient conditions at the time of mixing and application. If no induction time is indicated on the PDS, then induction is not required and the coating is ready for use. Some coating materials, such as zinc-rich primers or glass flake coatings require the addition and mixing of other “dry” components (blended into the liquid components) and may require straining (Figure 3) to remove any large particulates that are not wetted by the liquid component(s).
Finally, it may be necessary to thin or reduce the coating material prior to application (Figure 4). Many protective coatings are thinned to achieve a more consistent spray pattern and to produce a more uniform, continuous film. When thinning a coating, it is essential to use only the thinner materials that are listed on the manufacturer’s PDS. Substituting an unauthorized thinner or solvent other than what is listed can adversely affect the coating material due to incompatibility of the various chemical components, and reactive thinners can react with one or both of the coating components. If a material is thinned, it is important to thoroughly blend the thinner into the mixed coating material to ensure uniform distribution. The maximum volume of thinner that can be added is listed on the PDS, typically as a percentage (e.g., thin up to 10%). When preparing for application, one must be aware that, if thinner was added to the coating material, the percent volume solids of the coating will be effectively reduced and the target wet film thickness required for application will need to be adjusted accordingly. The adjusted volume solids content can be calculated using the following formula:
% solids by volume ÷ (100% + % thinner added)
While mixing a coating may seem like a simple task, the impact that it can have on coating performance is significant. Improper mixing can lead to an incomplete chemical reaction, resulting in reduced coating performance. By contrast, a proper mix will give a coating material the best chance to perform as intended. Every coating is designed by its manufacturer to be mixed and applied properly. Diligently following the manufacturer’s specifications for coating preparation will increase the likelihood of peak coating performance.