Consequences of Incorrect Mixing of Multi-Component Coatings

Many important industrial paints and coatings are two or three-component products (designated as two-component 2K and three-component 3K) in which a chemical reaction between components forms a coating film. To create the desired final product, separately packaged components must be mixed in a specific ratio. The compositions of materials in single containers are vastly different. The components undergo a chemical reaction when mixed together. In this way they create a cross-linked polymer material that forms a protective coating. The product data sheet (PDS or TDS) for purchased material contains the manufacturer's information regarding the correct mixing ratio of the components and mixing parameters. Although two and three-component coatings exist, this article will focus primarily on two-component materials as they are more commonly seen in the coating industry. In general, coating/paint manufacturers supply materials in two pre-measured cans, each containing the correct amount (ratio) of each component. It is identified with the correct component label and the base amount provided is mixed with the activator amount. This pre-measured packaging, commonly referred to as a mixing kit, allows the product to be used without needing to separately proportion each component. Nevertheless, even with pre-measured packaging, situations can occur where components are not mixed in the correct proportions. In some cases, it may be necessary to measure and proportion the individual components of a pre-packaged kit, because the materials are supplied only in large quantities (for example, 5-gallon kits) and the pot life may limit the amount of material that can be used in a single mix. Additionally, repair programs and requirements for repairs of small areas that may not require use of a complete kit may require proportioning. However, most coating manufacturers do not recommend (and may even prohibit) ratioing components for a product supplied as a packaged kit.

Correct Mixing Ratios

The manufacturer, particularly the coating formulator, determines the correct mixing ratio of coating components for a 2K product. In some coating types, such as polyurethanes and epoxies, even minor variations in the mixing ratio can adversely affect performance. Fast-curing coating products, commonly applied with automated multi-component spray equipment, are generally more sensitive to mixing ratio variations and small deviations can greatly alter the performance properties of the final coating. The permitted variation in correct mixing ratio depends on the coating type, formulation, and end-use properties. The correct mixing ratio relates to the chemical stoichiometry of the two coating components reacting together. There are two components, and each has a known quantity of chemically active sites that need to be chemically bonded to form the final film. For the reaction to occur optimally and polymerization to take place, the two components are chemically balanced when they have an equal number of available sites. This is another way of saying that all reactants are consumed without either the base or activator (or hardener) component being deficient or in excess. This optimal chemical balance does not always translate to a mixing ratio of one part base to one part activator, and sometimes not even to an intuitively proportioned ratio. Scientific studies have shown that the more intuitive the mixing ratio, the less likely the materials will be mixed properly; however, the human factor is being overlooked here. When working in high-temperature environments, the pot life of two-component materials may decrease, sometimes significantly. An application done in morning sunlight, on the other hand, may mean a noticeable reduction in the pot life of the coating. The fast reaction, due to the shortened pot life, leads to significant waste and cost and results in properly mixed but unusable coating material. For example, suppose an applicator attempts to preserve remaining material by applying only component A and omitting component B. Since component B is a relatively small portion (the mixing ratio was 20A:1B), only the remaining amount of the thicker component A is expected to adequately coat the surface as a continuous film and the heat to improve the coating. However, it will be found that the curing time is significantly prolonged. For this reason, a dry film thickness reading cannot be obtained because the coating does not cure sufficiently and subsequent coating layers cannot be applied.

What is Induction Time?

Some coatings require an induction period to ensure the curing reaction proceeds efficiently and correctly. Induction time is a period during which newly mixed material is allowed to stand before application, and is not present in all coatings. Information is provided on the application data sheet.

General Mixing

All coatings consist of resins, pigments, additives, and solvents. Heavier pigments generally settle to the bottom of the container and must be thoroughly mixed and homogeneously distributed before use. Sometimes resin or solvent may rise to the surface (a phenomenon called syneresis). This clear liquid is an important part of the paint formulation and should never be poured off - it must be thoroughly remixed before use.

Procedures for Mixing 2-Component Coatings

Mixing of single-component coatings can generally be adequately accomplished using flat, electric mixers, blade mixers, or paint shakers. Mixing of 2-component coatings (particularly those with high solids content) requires somewhat more effort. Proper mixing of 2-component coatings before application is an important step in the entire painting process. Many coating defects can be traced back to insufficient/incorrect mixing. There is one additional reason for correct mixing when it comes to 2-component coatings. The standard 2-component kit contains Part A and Part B; one contains the reactive resin while the other contains the curing agent or 'hardener'. [Some kits actually contain multiple packages; in addition to the resin and hardener packages, reactive color packages and accelerators are sometimes also supplied.] The ratio of resin to hardener must ensure that, when properly mixed, the two reach complete reaction in order to form the final film. Think at the microscopic level; every molecule must be thoroughly mixed with its reactive counterpart in order to react. Using an electric mixer, the speed and efficiency of mixing ensures, at the molecular level, that Part A and Part B are thoroughly mixed and that the reaction occurs correctly and uniformly throughout the mix.

If mixing is not sufficiently thorough, there may be excess amounts in either Part A or Part B, which results in incorrect mixing ratios in those areas. This is sometimes called 'disproportionate' mixing. Disproportionate mixing can cause any or all of the following:

• Uneven, lower gloss than specified on the data sheet, • Inconsistent hardness, areas that are too soft or too brittle (depending on product type), • Significantly lower hardness than specified on the data sheet, • Coating failure such as delamination, • Film defects such as blooming, • Non-adhesive or uncured, unreacted coating.

Effects of Ratio Mixing

Mixing the two components of a coating material at a ratio different from that specified by the manufacturer may result in changes to the final film properties. These changes, which may not be apparent during coating application, sometimes only become visible after the coating process is complete (Figure 1). However, most of the time defects resulting from disproportionate mixing are visually apparent during application (Figure 2). There is no single physical property that can be used as an indicator for all two-component materials that would directly show the effect of disproportionate mixing. For this reason, it is difficult to accept the identification of a disproportionate mix as the sole cause of an observed defect. [caption id="attachment_103656" align="aligncenter"] Figure 1. Three mixes of the same material designated at the same hardness on glossy surfaces. Photos courtesy of KTA-Tator, Incorporated[/caption] [caption id="attachment_103657" align="aligncenter"] Figure 2. Over-catalyzed material creates a rough surface by forming bubbles. Foaming occurred at the incorrect mixing ratio on the right side.[/caption]   When coating components are mixed disproportionately, the chemical reaction is not balanced and the presence of excess reactant can cause a visual change in the material. The most visually apparent and tactilely most characteristic condition of an uncured material is that the material has not reacted and remains essentially wet (Figure 3). This means runs, sags, and dirt and debris sticking to the soft, sometimes tacky surface of the unreacted material. Another example of an immediate visual change is cracking that can appear rapidly as the two components react and form a film harder than intended (Figure 4). In other cases, a noticeable change does not occur immediately even if the chemical reaction is not complete. For example, it appears from Figure 5 that all films produced by properly mixed and disproportionate mixes are similar. However, when the surface is examined, it is noted that the disproportionate mix on the left is much softer than the other two mixes. Whether or not visual variations exist, it is important to remember that the excess component remains in the film. The remaining material may or may not cause visual changes. In some cases, a film produced with a disproportionate mix may develop over a certain time period depending on the coating type, service environment, and also the degree of the material's mixing ratio deviation. [caption id="" align="aligncenter"] Figure 3. The disproportionate, low-catalyst mix on the right did not cure even after several months.[/caption]   [caption id="" align="aligncenter"] Figure 4. Low-ratio, low-catalyst mix on the left has cracked extensively. Magnification is shown at 20x.[/caption]   [caption id="" align="aligncenter"] Figure 5. Three mixes with similar gloss and appearance differed in hardness.[/caption]  

Incorrect Mixing by Coating Type Epoxy

An epoxy coating mixed at an improper ratio may produce a film that is visually indistinguishable from a properly mixed film. In this case, among some of the properties that can be altered by the out-of-ratio components are flexibility, chemical resistance, water resistance, and hardness. Depending on the service environment and the property affected by the disproportionate mixing, the defect may not be immediately noticeable. For example, an incorrectly mixed epoxy applied to a concrete substrate may still protect the substrate if the only resulting defect is reduced flexibility. However, the same incorrectly mixed epoxy coating applied to flexible steel flooring may fail at the first change in environmental conditions causing the floor to move or flex, and ultimately may cause delamination. Another example is an epoxy floor coating that looks good when first applied (Figure 6), but after the building is put into service, the coating is easily scratched. Eventually the coating had to be completely replaced, essentially for aesthetic reasons. Conversely, if an out-of-ratio epoxy material was applied to provide chemical resistance, the poorly mixed film may soften, discolor, or dissolve when exposed to chemicals, thus failing to provide chemical protection to the underlying substrate. Additionally, even if the coating appears acceptable, application of a topcoat containing solvents may weaken an underlying epoxy intermediate or primer and lead to future delamination. If the applied epoxy is hardened with amine, excess amine may cause amine bleed to the surface (blooming), which can lead to delamination of the topcoat. [caption id="attachment_103661" align="aligncenter"] Figure 6. Reduction in wear resistance of epoxy floor coating due to disproportionate mixing is viewed at 50x magnification.[/caption]  

Polyurethanes

Two-component polyurethane materials mixed at improper ratios tend to produce softer films that are prone to discoloration or showing differences in gloss. Figure 2 shows that excess component B in the coating causes the formation of voids and bubbles. Besides immediately visible defects, solar radiation energy causes early degradation of the poorly mixed polyurethane resin, color fading, and loss of gloss. In addition, the coating may not adhere properly to the substrate or underlying materials. Some of the raw materials used to formulate polyurethane materials may be moisture-sensitive, so incomplete reaction of these components may cause adverse effects on the water resistance of the coating. Increased moisture sensitivity may, similar to reactions that may occur if properly mixed polyurethane coatings are prematurely exposed to water, further alter the properties of the final film through increased voids and water erosion.

Polyureas

There are many different polyurea resins, polyurea resin blends, and hybrid formulations. The faster reaction rate of the two components of these coatings, compared to other multi-component coatings, makes them more sensitive to disproportionate mixing. Because there are so many formulation variables, all the defects listed above for both epoxy and polyurethane materials also apply to polyurea materials. The mixing ratios of these materials can vary greatly and proper and uniform operation of the application equipment is required for the components to be properly mixed. Normally mixing occurs at the spray gun tip and is completed between seconds and several minutes. When products react to this, there is a higher degree of sensitivity for the complete introduction and mixing of the two components. The end film of out-of-ratio material may have physical evidence of incorrect mixing, including swirls and streaks of unreacted material (Figure 7). In other cases, the material may show no adhesion to the underlying surface. Other coating failures may be recorded after the structure is put into use. Because the materials typically rapidly produce hard, chemically resistant films, it is difficult to determine what the service life of an inadequate film would be unless testing is performed. [caption id="" align="aligncenter"] Figure 7. A colorless coating due to disproportionate mixing of a multi-component material, magnified at 20x.[/caption]  

Examination of Mixing Ratio Variations Solvent Sensitivity

One of the fastest and easiest ways to examine two-component materials for mixing ratio variations is a solvent rub test. The most common method for assessing solvent sensitivity is ASTM D5402, Standard Practice on Evaluating Solvent Resistance of Organic Coatings Using Solvent Rubs. This method involves saturating a cloth with an appropriate solvent and rubbing the surface of the coating. Most product data sheets list the expected solvent resistance value of a coating. If the final film does not meet the required number of double rubs, the two components potentially were not mixed correctly. Depending on the coating type and application and curing conditions, this method can typically be performed within 12 to 48 hours after application.

Analytical Methods

Several analytical methods can also be used to determine the mixing ratio of coating materials after application. Most of these methods require liquid coating samples of the components. The components are then mixed in the proper ratio and re-mixed at ratios representing over-catalyzed and under-catalyzed mixes. The dried films are then analyzed to plot a 3-point data curve that can be used to compare data obtained from the coating material alleged to be improperly mixed and from control samples. There are various techniques that can be used to obtain confirmation of the mixing ratio using laboratory-prepared samples. Three of these techniques, namely infrared spectroscopy, differential scanning calorimetry, and nitrogen content, are examined below.

Infrared Spectroscopy

Samples analyzed with infrared spectroscopy produce infrared spectra that can be interpreted to determine the ratio of peaks representing the resin and pigment portions of the material, provided that pigment components are specific to one of the two components. In addition, reaction components and reaction products can also be used to track the curing of the two components.

Differential Scanning Calorimetry

Samples analyzed with differential scanning calorimetry (DSC) produce data curves used to define the glass transition temperature of the material. Glass transition temperature is affected by the cross-linking of the cured coating. A cured coating product exhibits a specific glass transition temperature. If the two components are not mixed in the correct ratio, the glass transition temperature will be different. For example, an under-catalyzed material will not achieve the cross-link density of a properly mixed product and will subsequently have a lower glass transition temperature than expected. Conversely, when the same material is over-catalyzed, the cross-link density is greater than that of the properly mixed material and leads to a higher glass transition temperature. Material with higher glass transition temperature typically is associated with a more brittle film that could be prone to cracking.

Nitrogen Content

Samples analyzed for nitrogen content are limited to coating products where only one of the components contains a nitrogen-containing material. For example, unreacted isocyanate in polyurethane coatings can be measured. Control samples (correctly mixed, over-catalyzed, and under-catalyzed) are used to determine the change in nitrogen content when there is the correct amount of nitrogen in the correctly mixed product as well as excess and deficient amounts of isocyanate. Nitrogen content typically exhibits a linear relationship between excess and deficient amounts of isocyanate (depending on the mixing ratio). The linear relationship can be used to back-calculate the mixing ratio of the coating applied to the field when tested using the same technique. Recommended instruction sequence for mixing 2-package materials: • The availability of cleaning solvent should be confirmed before starting application. • The contents of each container should be thoroughly mixed with a power mixer. • The sides and bottom of the container should be progressively scraped, and a flat-bladed knife can be effective in accomplishing this. • Ensure the bases are painted the correct color. • All containers should be containerized before use to ensure color consistency. • Under constant mixing, components should be added in the recommended order. This usually involves one person pouring while another mixes. • Using a power mixer according to the manufacturer's specifications, the contents of both packages should be thoroughly mixed. The sides and bottom of the container should be scraped to ensure all 'Part A' and 'Part B' are mixed. Perfectionists often transfer the mixture to a separate, clean container to prevent contamination of unreacted material. While this is an appreciated practice, it can generally be said that this is usually unnecessary if the sides and bottom of both containers are scraped and effectively mixed. The mix should be allowed to stand for the recommended induction period as stated on the product data sheet.

Conclusion

The effects of mixing a two-component material at a ratio different from that specified by the manufacturer can result in a wide range of failures. Some of these defects can result in coating failure. The defects depend on the type of material mixed, the degree of deviation from the correct ratio, environmental conditions during mixing, and any effect of the substrate material. If there is suspicion that components have been mixed out of ratio, in addition to screening tests that can be used to assess field coatings, laboratory tests are available that can provide more definitive evidence of out-of-ratio mixing of the two components. References 1. KTA University, Incorrect Mixing of Multi-Component Coatings, Accessed: 5 March 2018, https://ktauniversity.com/incorrect-multi-component-coatings/ 2. http://www.dacrylate.co.uk/Hub/2pckx.html   Seda Ömercikoğlu Yüce, MICorr, PCS NACE Senior Corrosion Technology Specialist General Manager STM Coatech