Many important industrial coatings are two- or even three-component products (also referred to as two-pack, 2K, or three-pack, 3K) in which a chemical reaction between the components results in a coating film. The components are packaged separately and require mixing at a specific ratio to form the desired final product. The compositions of the materials in the individual cans are very different. When the components are mixed together, they undergo a chemical reaction resulting in a cross-linked polymer material that forms a protective coating.
The product data sheet for the purchased material provides the correct mixing ratio of the components and the manufacturer’s information concerning mixing parameters. While there are two- and three-component materials, this column will focus primarily on two-component products because they are more prevalent in the coatings industry.
Typically, coating manufacturers supply the materials in two premeasured containers that contain the correct amount (ratio) of each component. The proper component is identified by its label, and the supplied amount of the base is mixed with the supplied amount of the activator. The premeasured packaging, usually referred to as a mixing kit, permits use of the product without the need to separately proportion each component in the shop or field. Yet, somehow, even with the premeasured packaging, there are occasions when the components are not mixed in the correct proportions. In some cases, the measuring and proportioning of individual components of a pre-packaged kit may be required because the materials are supplied only in large quantities (e.g., 5-gallon kits), and the pot life may limit the amount of material that can be used in a single mix. Additionally, maintenance schedules and requirements to repair small areas, which may not require use of a full kit, may necessitate proportioning. Most manufacturers do not recommend (and may even prohibit) proportioning of components for a product supplied as a packaged kit.
Correct Mixing Ratio of Coating Component
The correct mixing ratio of coating components for a 2K product is determined by the manufacturer—specifically, the coating formulator. Some slight variation of the mixing ratio may not necessarily negatively impact the performance of some coating types, such as polyurethane and epoxy materials. Rapid-cure coating products commonly applied by plural-component spray are often more sensitive to mixing ratio variations, and the performance characteristics of the final coating can vary greatly with small deviations. The allowable variation from the proper mixing ratio depends on the type of coating, the formulation, and the end-use properties.
The correct mixing ratio is related to the chemical stoichiometry of the two reacting coating components. There are two components, and each has a known amount of chemically active sites that will be needed to chemically bond to form the final film. The reaction is chemically balanced when the two components have an equal number of available sites for the reaction to optimally occur and form the polymer. This is another way of saying that all of the reagents are consumed with no deficiency or excess of either the base or the activator component. This optimal chemical balance may not always translate to a mixing ratio of one part base to one part activator, or even a somewhat convenient ratio of two parts base to one part activator.
Scientific studies have revealed that the more intuitive the mixing ratio, the less likely the materials are to be mixed improperly; however, there is no accounting for the human element. When working in heated environments, the pot life of two-component materials can be shortened, sometimes considerably. A situation occurred when an application in the morning sun resulted in a significant reduction in the pot life of the coating. The fast reaction had caused substantial waste and cost due to the shortened pot life, resulting in properly mixed but unusable coating material. The applicator attempted to conserve the remaining material by applying only component A and omitting component B. Because component B was a relatively small portion (the mixing ratio was 20A:1B), it was believed that the remaining quantity of the thicker component A alone would sufficiently cover the surface as a continuous film, and that the heat would cure the coating. Needless to say, the cure time was extended indefinitely. A dry film thickness reading could not be obtained because the coating did not harden sufficiently, and the subsequent coating layers could not be applied.
Effects of Off-Ratio Mixing
If the two components of the coating material are mixed at a ratio that differs from the ratio specified by the manufacturer, then a change in the final film properties can occur. Sometimes these changes are not evident while the coating is being applied and show up only after the coating is placed into service (Fig. 1). In other cases, the defects caused by off-ratio mixing are visually evident during application (Fig. 2). There is not a single, direct physical property that is affected by off-ratio mixing that can be used as a marker for all two-component materials. Therefore, identifying an off-ratio mix is difficult to recognize as the sole reason for the observed defect.
|Fig. 1: Three mixes of the same material all cured to glossy finishes with the same hardness. Photos courtesy of KTA-Tator, Inc.|
|Fig. 2: Over-catalyzed material formed bubbles, creating an uneven surface. Foaming occured at the incorrect mixing ratio on the right side.|
When the coating components are mixed off-ratio, the chemical reaction is out of balance, and the presence of excess reactant can cause a visual change in the material. The most visually apparent and tactile cases result in a coating material that does not cure; the material remains unreacted and, essentially, wet (Fig. 3). This can lead to runs and sags and the adherence of dirt and debris to the soft, sometimes sticky surface of the unreacted material. Another example of an immediate visual change is cracking, which can occur quickly, as the two components are reacting and forming a film that is more rigid than the intended coating (Fig. 4). In other cases, even if the chemical reaction is not complete, there is no immediate visible change. For example, in Figure 5, all of the films produced by the proper and off-ratio mixes appear similar. Probing the surface revealed that the off-ratio mix on the left side was much softer than the other two mixes. With or without the visual variations, it is important to remember that the component that is in excess remains in the film. The remaining material may or may not lead to changes in appearance. In some cases, the film produced by an off-ratio mix may perform for a certain period of time, depending on the coating type and the service environment, as well as the degree to which the material was mixed off-ratio.
|Fig. 3: Off-ratio, under-catalyzed mix on the right side did not cure, even after several months.|
|Fig. 4: Off-ratio, under-catalyzed mix on the left side cracked abundantly. The inset is shown at 20x.|
|Fig. 5: Three mixes that were similar in gloss and appearance displayed variations in hardness.|
Evidence of Mis-Mixing by Coating Type
An epoxy coating mixed at an improper ratio may form a film that is hard and not visually different from a film mixed at the proper ratio. In this case, some of the properties that may be altered by components that are mixed off-ratio include, but are not limited to, flexibility, chemical resistance, water resistance, and hardness. Depending on the service environment and the property affected by the off-ratio mixing, the defect may not be immediately noticeable. For example, an improperly-mixed epoxy applied to a concrete substrate may still protect the substrate if the only resultant defect is a decrease in flexibility. However, the same improperly-mixed epoxy coating applied to flexible steel decking may fail and eventually delaminate at the first change in environmental conditions that causes the decking to move or flex. Another example is the epoxy floor coating (Fig. 6) that appeared to be fine when it was first applied, but after the building was occupied, the coating was easily scratched. Eventually the coating had to be completely replaced, mainly because of aesthetics. Conversely, if the off-ratio epoxy material was applied to provide chemical resistance, the mis-mixed film may soften, discolor, or go into solution upon exposure to chemicals, thereby providing no chemical protection to the underlying substrate. Additionally, even if the coating appears acceptable, application of a top-coat containing solvents may weaken an underlying epoxy mid-coat or primer and lead to future delamination. If the epoxy being applied is an amine-cured material, the excess amine could lead to an amine exudate (blush) on the surface, which can result in delamination of the topcoat.
|Fig. 6: Decreased abrasion resistance of an epoxy floor coating due to off-ratio mix, viewed at 50x.|
Two-component urethane materials that are not mixed at the proper ratio tend to form softer films that are prone to discoloration or may display variations in gloss. Figure 2 shows how the excess component B leads to the formation of voids and bubbles within the coating. Apart from immediate visible defects, the energy produced by solar radiation can result in early deterioration of the improperly mixed urethane resin, causing color fading and loss of gloss. Additionally, the coating may not develop proper adhesion to the substrate or underlying materials. Some of the raw materials used to formulate urethane materials can be water sensitive, so the incomplete reaction of these constituents can have detrimental effects on the water resistance of the coating. The increased water sensitivity can further change the properties of the final film, with increased occurrences of voids and water erosion, similar to the reactions that can occur when uncured, properly-mixed urethane coatings are exposed to water prematurely.
There are numerous types of polyurea resins, blends of polyurea resins, and hybrid formulations. The rate of reaction of the two components of these coatings makes them seem to be more sensitive to off-ratio mixing than other multi-component coatings. Because there are so many formulation variables, all of the defects listed above for both epoxy and urethane materials also apply to polyurea materials. The mixing ratios of these materials can vary widely, and the application equipment must be accurate and functioning properly for the components to blend properly. Mixing normally takes place at the gun tip and is complete within a matter of seconds to several minutes. When products react this quickly, there is a greater degree of sensitivity (precision) for complete introduction and mixing of the two components. The final film of off-ratio material may have physical evidence of mis-mixing, including swirls and streaks of unreacted materials (Fig. 7). In other cases, the material may not develop adhesion to the underlying surface. Other coating defects may be noted after the structure is placed in service. Because the materials normally develop hard, chemical-resistant films immediately, a deficient film may be difficult to identify early in its life cycle unless testing is conducted.
|Fig. 7: Discolored coating due to off-ratio mix of plural-component material, magnified at 20x.|
Examining Coatings for Mixing Ratio Variations
The quickest and easiest way to examine some two-component materials for mixing ratio variation is with a solvent rub test. The most common method to assess solvent sensitivity is ASTM D5402, Standard Practice for Assessing the Solvent Resistance of Organic Coatings Using Solvent Rubs. This method involves saturating a cloth with a suitable solvent and rubbing the surface of the coating. Most Product Data Sheets list a coating’s expected solvent resistance value. If the final film does not meet the required number of double rubs, then the two components potentially were not mixed correctly. This method can be performed in the field, and, depending on the coating type and conditions of application and curing, usually within 12 to 48 hours after application.
There are several analytical methods that also can be used to determine the mixing ratio of coating materials after they have been applied. Most of these methods require liquid coating samples of the components. The components are then mixed at the proper ratio and mixed again at ratios representing an over-catalyzed and an under-catalyzed mix. The dried films are then analyzed to graph a 3-point data curve, which can be used to compare the data obtained from the allegedly mis-mixed coating material and the control samples. There are several techniques that can use laboratory-prepared samples to arrive at a mixing ratio confirmation. Infrared spectroscopy, differential scanning calorimetry, and nitrogen content are three of the techniques that will be reviewed below.
Samples analyzed by infrared spectroscopy generate infrared spectra, which can be interpreted to determine the ratio of peaks representing the resin and pigment portions of the material, provided the pigment components are exclusive to one of the two components. Additionally, reaction materials and the reaction products may also be used to follow the cure of the two components. An example of this technique is presented for an epoxy material in Figure 8.
|Fig. 8: Epoxy mixing ratio variations|
In the figure, the peak at 1,506 is from the epoxy component, and the peak at 1,454 is due to the curing agent (component B). Because the ratio of these bands varied with the mixing ratio for this material, the ratio of these peaks can be used to show proper or improper mixing. For example, note that the 1,508 cm-1 band is larger than the 1,454 cm-1 band in the first mixing ratio (1A:1B), and the 1,454 cm-1 band decreases in size relative to the 1,508 cm-1 band as the amount of component B is decreased, showing that the coating is off-ratio. This technique is useful if the materials have an initial mixing ratio of 1:1, 2:1, and 4:1. As the amount of the activator decreases beyond the 4:1 mixing ratio, the mixing ratio variations may not be evident by this technique.
Similarly, Figure 9 represents the use of infrared spectroscopy to determine the mixing ratio of a urethane material. In this case, the two bands used for the evaluation were the 1,730 cm-1 band (resin) and the 1,690 cm-1 band (catalyst). As the amount of catalyst is increased, the 1,690 cm-1 band increases in size relative to the 1,730 cm-1 band. Although these examples show clear mixing ratio variations for these products, these band pairs and this technique may not work to examine mixing ratio variations in all epoxy and urethane materials.
|Fig. 9: Urethane mixing ratio variations|
Differential Scanning Calorimetry
Samples analyzed by differential scanning calorimetry (DSC) produce data curves that are used to define the glass transition temperature of the material. The glass transition temperature is affected by the degree of cross-linking of the cured coating. A cured coating product exhibits a specific glass transition temperature. An example of a classic glass transition temperature of a properly-mixed material is portrayed in Figure 10. If the two components are not mixed at the proper ratio, the glass transition temperature will change. For example, an under-catalyzed material cannot achieve the cross-link density of the product mixed at the proper ratio, and subsequently, the glass transition temperature would be lower than expected. Conversely, when the same material was over-catalyzed, the cross-link density was greater than the properly-mixed material and resulted in a higher glass transition temperature. The higher glass transition temperature material is often associated with a more brittle film, which may be prone to cracking.
|Fig. 10: DSC of properly-cured epoxy|
Samples analyzed for nitrogen content are limited to coating products in which only one of the components contain a nitrogen material. For example, the unreacted isocyanate of urethane coatings can be quantified. Control samples (mixed at the correct ratio, over-catalyzed, and under-catalyzed) are used to determine the amount of nitrogen in the product mixed at the correct ratio, as well as the change in nitrogen content when excess and deficient amounts of isocyanate are present. The nitrogen content typically exhibits a linear relationship between deficient and excess amounts of isocyanate (based on the mixing ratio). The linear relationship can be used to back-calculate the mixing ratio of the field-applied coating when tested using the same technique.
The effects of mixing a two-component material at a ratio that is different from the ratio specified by the manufacturer can cause a wide range of defects, and some of these defects can lead to coating failure. The defects are based on the type of materials being mixed, the degree of deviation from the correct ratio, environmental conditions during mixing, and any influence of the substrate material. If off-ratio mixing of components is suspected, there are screening tests that can be used to evaluate the coating in the field, as well as subsequent laboratory tests that can provide more definitive evidence of an off-ratio mix of the two components.
|Valerie Sherbondy is the Technical Manager for the Analytical Laboratory for KTA-Tator, Inc., a consulting and engineering firm specializing in industrial protective coatings. Ms. Sherbondy has been employed at KTA since 1990 and has provided laboratory support for the investigation of hundreds of coating failures and coating testing programs. She holds a B.S. in chemistry from the University of Pittsburgh and is an SSPC-Certified Protective Coating Specialist, a member of the American Chemical Society (ACS), and a committee chair for NACE International.|