KTA’s Certified Coating Inspector Forum Volume 3, Issue No. 9 – September 2024
William Corbett, COO
AMPP Senior Certified Coating Inspector & Certified Protective Coating Specialist
Valerie Sherbondy, Director of Analytical Laboratory Services
AMPP Certified Protective Coating Specialist
KTA’s Certified Coating Inspector Forum is designed to provide professional development/continuing education on standards, inspection practices, new instruments, and other topics to help keep certified AMPP and FROSIO coating inspectors current. It represents the views of the author and KTA-Tator, Inc. It may or may not represent the views of AMPP: The Association for Materials Protection & Performance, even though SSPC, NACE, and AMPP standards are frequently referenced in the content.
Introduction
For this edition of the KTA Certified Coating Inspector Forum I have teamed with Ms. Valerie Sherbondy, the Director of KTA’s Analytical Laboratory Services. Valerie has over 35 years of coating industry experience and has been with KTA for over 30 of those years. More impressively, Valerie has diagnosed hundreds of coating failures during her career, including those caused by the presence of amine blush/bloom that was not addressed prior to recoating. Together, we describe amine and why it is used to formulate certain coatings, identify the conditions that are conducive to amine exudate[1] (blush) formation, describe the chemical reaction of amine with various sources of reactants and the appearance/consistency of amine blush on coated surfaces, and explain the consequences, detection, remediation, and prevention of amine blush formation.
Amine and Its Use in Protective Coatings
Coating chemistry and formulation can be complex, but here’s a brief explanation of amine and its use in protective coatings.
Amine is a reactant and therefore commonly found in the activator or “Part B” of an epoxy coating. While other coating types may use amine, the primary use is in epoxy coatings. Epoxies are tightly cross-linked, provide excellent chemical resistance, and have excellent performance in immersion service. They are frequently referred to as the “workhorse of industrial coatings.” Epoxy coatings are compatible with many other coating types and as such are used as part of a variety of coating systems, e.g., zinc primer/epoxy mid-coat/polyurethane finish coat. Epoxy may also be used in the formulation of other coatings you may encounter, e.g., organic (epoxy) zinc primer, which contains epoxy as the resin/binder, or polysiloxane, which combines epoxy and polyurethane for improved performance as a finish coat.
Depending on formulation, amines can also be used to adjust the viscosity of a coating (thereby incorporating less solvent for VOC regulatory compliance), as a wetting agent to improve flow-out once applied to a surface, and to adjust pH for better compatibility with other coatings and the substrate.
Generically an amine is any member of a family of nitrogen-containing organic compounds. It is derived from ammonia (NH3). There are four variations of amine:
- Primary Amines contain one hydrogen replaced by an alkyl or aryl group, such as methylamine.
- Secondary Amines contain two hydrogen atoms replaced by an alkyl or aryl group, such as dimethylamine and diphenylamine.
- Tertiary Amines contain three hydrogen atoms replaced by an alkyl or aryl group and can start using an aromatic ring (trimethylamine).
- Cyclic Amines are secondary or tertiary amines in an aromatic ring structure.
Coatings that contain tertiary and cyclic amines (that have an aromatic ring structure) are more prone to color shift and gloss reduction when exposed to solar radiation (sunlight).
Conditions Conducive to Amine Blush Formation
Conditions that can cause the formation of amine blush include cold, damp ambient conditions during coating application and cure, condensing moisture (typically caused by a surface temperature at/below dew point temperature during coating cure), high relative humidity during coating application and cure, as well as reaction with other gases.
Chemical Reaction of Amine with Water and Other Reactants
An article posted to KTA University several years ago contained an illustration of the chemical reactions associated with the formation of amine blush.
While we often focus on moisture as the culprit, other reactants can include carbon monoxide (e.g., diesel exhaust from equipment), carbon dioxide, and the use of heaters to control temperatures. But if the amine is designed to react with the epoxy, then why would it react instead with these other sources? This occurs because the reaction between the amine and the epoxy is not the ‘easiest’ path. Anything present that will preferentially react with the amine or the –NH2 group, shown above (i.e., easier than the reaction with the epoxy) is of concern.
Note that adherence to any required induction time, also called sweat-in, digestion, and cook time, is critical to ‘encourage’ the epoxy-amine reaction, versus other reactant sources. Induction times, when required are listed on the coating manufacturer’s product data sheet and may be material temperature dependent (i.e., longer induction times for cooler temperatures and shorter induction times for warmer temperatures).
Appearance/Consistency of Amine Blush on Freshly Coated Surfaces
Initially, the amine blush appears as a milky off-white/pale yellow haze. Depending on the color of the epoxy coating (and available lighting) it may be difficult to see. However, to the touch amine blush has an oily or greasy consistency. As the amine blush oxidizes it turns from nearly translucent to yellow, to tan, and then light brown. In severe cases it may become dark brown. It also changes from the oily/greasy consistency to tacky, and eventually a dry film. The more the amine blush oxidizes the more challenging it can be to remove.
Consequences of Overcoating Amine Blush
If the formulation of the next layer is surface tolerant (and the amount of amine blush is very minor), there may be no consequences of coating over it. However, “surface tolerant” is more a marketing phrase than a technical one and is also relative. Coating over amine blush is a high-risk proposition and is not recommended. Written approval from the coating manufacturer’s technical representative is highly encouraged when overcoating is being contemplated.
If the amine blush is at the “oily/greasy” stage, it will act as a bond-breaker, causing catastrophic adhesion loss from the underlying coating layer, (the layer that “blushed/ bloomed”). For immersion or splash service, osmotic blistering can occur as the water passes through the overcoat and reacts with the underlying amine (amines are highly soluble in water).
Field Detection Methods
As of this writing there are two kits on the market that are designed to detect amine on the surface. One kit relies on elevated surface pH to indicate amine while the other kit detects the presence of ammonium carbamate (shown in the chemical reaction figure earlier in this article).
The TQC Sheen Test Kit (shown right) is based on indication of amine via surface pH. It is quick and easy to use but since it indicates by surface pH there is the potential for a false positive (i.e., something other than amine blush is on the surface that is causing a higher pH indication). The kit includes a 50mL nebulizer bottle of indicator solution, 100 visualization (filter) papers, tweezers, a color gradient chart, and a set of directions.
Briefly, a piece of filter paper is saturated with the indicator solution and placed onto the suspect surface, then the color compared to the color gradient chart.
The far-left filter paper sample is the control, which illustrates the color the filter paper becomes when the indicator solution is applied. The filter paper sample labeled “OS” was placed onto surface of an amine-cured epoxy and revealed a purple coloration, which according to the color gradient chart is a pH 9 and is considered “highly contaminated.” Filter paper samples “BC” and “ER” were placed onto the same coating material that was cured either under different conditions or for a longer duration. Sample “BC” indicated amine is “likely” present, and Sample “ER” indicated an “amine free” surface when compared to the color gradient chart.
The Elcometer 139 Amine Blush Swab Test Kit (shown right) is based on the detection of carbamate. While there a very low chance of a false positive, use of the kit can be complicated and time consuming. That is, it can take a couple of hours to obtain a single result due to reaction time requirements of the various indicator chemicals. The kit includes several small bottles of various solutions (some of which need to be prepared), swabs, tubes with caps for retaining the solutions, pipettes, tweezers and scissors, and a set of directions.
It is beyond the scope of this article to list the step-by-step instructions for the proper use of this kit, but careful adherence to the directions is paramount to obtaining a reliable result.
The four samples shown to the right were collected from the same test surfaces as those used with the TQC Sheen test kit. Since the color of the 3 samples (OS, BC, E) is the same as the control (left, labeled ‘Blank’), no carbamate is present, which indicates that a false positive for amine blush may have occurred when using the kit that indicates by surface pH.
Laboratory Detection
Given the limitations of the field test kits, the most reliable method of determining the presence of amine blush is in the laboratory using an analytical technique known as Infrared Spectroscopy, or IR. The suspect area can be sampled using a cotton ball or swab subsequently placed in a Ziplock bag and labeled. Upon receipt in the laboratory the sample is extracted in water (amine blush is very water soluble), collected in a mortar, and the water permitted to evaporate. The residue is combined with potassium bromide (KBr, which is transparent in the infrared region), examined by IR and the resulting spectrum interpreted for the presence of carbamate.
Remediating Amine Blush
Once amine is detected by either field or laboratory methods, the coating manufacturer should be consulted regarding remediation procedures, as remediation is often condition dependent. That is, if conditions were ‘slightly’ out of range and the amine blush is only minor, the surface may only require a water wash and dry, followed by recoat as scheduled. When the blushing is more severe, physical scrubbing of the surface may be necessary. Adding ethyl alcohol to the water may help solubilize the blush more readily, and hot water will work better than cold water. While detergents may be added to the water, they may be slightly alkaline (higher pH), which may generate a false positive if the surfaces are retested with the TQC Sheen test kit. If a positive high pH reading occurs after detergent cleaning, thoroughly rinse and wipe a small area with water, then retest to determine whether the detergent is a contributing factor.
For service-critical applications, if complete crosslinking (cure) of the epoxy is in question (because the amine reacted preferentially with water or a gas), removal and reapplication of the coating may be required.
Methods to Prevent Amine Blush Formation
As certified coating inspectors, our job is to anticipate potential issues and do what we can to help prevent them from occurring. In the case of amine blush formation, here are four things that can help:
- Know the temperature and humidity ranges/maximums listed on the product data sheet (PDS). For epoxy coatings there is often additional information on the potential for amine blush formation in the section that details application conditions or drying/curing. The January 2023 edition of the KTA Certified Coating Inspector Forum focused on how an inspector can benefit from locating and comprehending specific content on a PDS, and how specifications sometimes invoke requirements listed on the PDS.
- Measure ambient conditions & surface temperature prior to mixing. If conditions are conducive to amine blush formation, then there is no point in starting the mixing process. The contractor will need to wait for conditions to conform or condition the space, when permitted by the contract.
- Monitor ambient conditions & surface temperature at intervals throughout the application. Conditions can change/decline at any time. Never assume that conditions at the time of mix are stable.
- Review the placement of any heating equipment or potential sources of reactants such as exhaust from compressors that may react preferentially with the amine.
It is also important to know what the coating should look like and how it should feel once it is properly cured. The coating manufacturer’s technical representative may need to be consulted if you are unfamiliar with these characteristics for a given coating material.
Summary
This issue of the KTA Certified Coating Inspector Forum described amine and why it is used in coating formulations (and which coatings typically contain amine). It illustrated how amine blush is formed and listed the conditions that are conducive to its formation. It described the chemical reaction of amine with different sources of reactants, the appearance of amine exudate (blush) on coated surfaces, and the consequences of overcoating amine blush. It also identified various methods of detection in the field and laboratory, described remediation procedures, and listed four methods to help prevent its formation.
[1] An exudate is considered something the “exudes” from a surface. In the case of amine, the terms exudate, blush, and bloom are used synonymously.