Amine Blush and Amine Bloom of Epoxy Binder Systems in Protective Coatings can present a challenge. Blushing can occur when epoxy binder systems are applied under cold / humid conditions. Amine blush is a negative side effect of the normal curing reaction which can appear on the coating or flooring surface. This problem can appear in various forms; gloss reduction; grey cloudiness; a greasy / waxy layer or as white patches or crystals. In some instances it amine blush may not be immediately obvious. Unfortunately, on contact with humidity or moisture, the coated areas can show tell-tale opaque white marks. These white discolourations can appear during cure or even after the cure is completed. Although the white discolouration that appears is water soluble, it won’t in reality wipe off easily with water, thinners or solvents. As a result, the only way to remove it is by dry or wet sanding after the binder has cured completely.
What is Amine Blush / Amine Bloom?
Amine Blush and amine bloom are two subtly different processes. Amine Blush, sometimes called “water spotting”, happens when moisture from the atmosphere condenses on a coating surface during curing. Occasionally the moisture originates from within the porous substrate itself. Amine bloom presents itself as white patches, or as a milky appearance in clear coatings. This can result in a lack of a glossy finish in pigmented coatings.
Amine Bloom, or leaching, is rather different from amine blush. Amine bloom occurs when the quantity of condensate causes the water-soluble compounds (e.g. in garage floor paint) the to leach from the body of the floor paint to the surface. When the moisture on the surface evaporates, the components that have migrated out will form on the surface as sticky deposits.
In severe cases, amine blush or amine bloom can also cause even more noticable surface irregularities. These obvious aesthetic defects are significant enough to cause even more light scattering and this results in a even more whitened appearance.
Effects of Amine Blush and Amine Bloom
Amine Blush and amine bloom are surface defects that must be avoided in an epoxy garage floor coating. These amine effects alter the coating performance, resulting in discoloration over time (yellowing), poor gloss retention, reduced overcoatability and inter-coat adhesion. The most significant of these negative effects is the reduction in “overcoatability”. This is where there is not enough adhesion when a subsequent coating layer is applied to the system due to its surface energy modification. When this issue occurs with a final topcoat layer, the visual appearance can be worse due to the alteration in its mechanical and chemical properties. A related phenomenon to overcoatability issues can happen at the coating-substrate interface. If moisture containing dissolved CO2 diffuses out of a porous substrate such as concrete, it can prevent the surface curing properly at the interface. Incomplete curing then negatively impacts final adhesion. Amin Blush and amine bloom can be removed, but this adds time and expense to the concrete floor paint job.
Causes of Amine Blush and Amine Bloom
Blushing, and in some cases, blooming are chemical reactions. The curing agent is the chemical component of the epoxy binder system and it is this that causes the blushing or blooming. The curing agent, Low-molecular-weight (primary) amines are usually hygroscopic and often react with CO2 and moisture in the atmosphere to create an ammonium carbamate according to the following simplified equation:
A greasy, waxy layer that sometimes appears on epoxy binder systems are mainly the salts of ammonium (bi-) carbonate. Amine compounds on the surface combine to varying degrees with CO2 (depending on the type and formulation of the binder system), and moisture in the air to form hydrates of amine carbonate.
The overall result of these side reactions is that amine (active hydrogen) compounds, which are supposed to react with the epoxide compounds, are being used up and consequently, not all epoxide groups react properly with the curing agent, chemical bonding is compromised and the net result is under curing.
Major Contributing Factors
There are several contributing factors that play a role in the appearance of amine blush or amine bloom. In particular, two significant factors are most influential in the formation of carbamate or carbonate – the humidity and the reaction rate of the amine with CO2 relative to the epoxy-amine reaction rate.
The Effect of Humidity (Condensation)
The humidity percentage in the atmosphere changes all the time, varying even within small areas such as within one industrial unit. If substrate temperature falls below the critical dew point, what will happen is that the moisture in the atmosphere will begin to condense on the substrate. When the coating is applies, the air from the spray gun and subsequent solvent evaporation lowers the surface temperature of the epoxy coating. This effect is greatly accentuated when rapidly-drying solvents or a solvent mixture which has not been balanced properly is employed. Condensed moisture will elicit an amine blush reaction.
As a rule of thumb within the coatings industry, the lowest surface temperature that is acceptable for applying epoxy systems is 3 °C above the dew point. This consideration changes slightly when using coatings that are specifically formulated to be “moisture tolerant”.
Amine Bloom or leaching can happen in systems that contain water-soluble ingredients. Also included here are curing agents and additives that are components of solvent-based or water-based coatings. Upon exposure of Epoxy Binder Systems to high humidity, the components can leach onto the surface. Here they can often dissolve in the moisture and result in a shiny or oily-looking surface deposit. In less severe cases, if the deposit is still liquid, it can usually be washed off with ordinary soap and water. On the other hand, if the amine bloom is too severe and it saturates the surface before it is completely dried, discolouration, uneven gloss, drips, runs and even destruction of the binder can result.
The Effect of Reaction Kinetics (Rate of Cure)
Amine blush and Amine bloom relate to the reaction kinetics of an amine curing agent with moisture and/or CO2 in the atmosphere. The amine should react with epoxy resin rather than with moisture or CO2. It is obvious that amine blush or amine bloom effects may increase when the rate of the epoxy-amine reaction is reduced.
As the temperature falls, reaction rate of the amine and epoxy resin is significantly declines. For around every 10 °C fall in curing temperature, the reaction rate will drop by a factor of two. The fall in the curing rate of the epoxy-amine reaction gives the (primary) amines an increased time to leach to the surface and take part in additional competing reactions. During this curing period, if the atmosphere is damp and cold, the reaction of the amine with CO2 and water is more likely and this will result in the formation of amine blush and amine bloom.
Curing Agent Type
The type of curing agent is another significant factor that playing a role in whether amine blush or bloom might occur. The tendancy for blushing or blooming to occur correlates to the structure of the amine. Low-molecular-weight (cyclo) aliphatic amines, (usually used in combination with epoxy resins), are usually hygroscopic and have a very high vapor pressure. These types of products are prone to blushing or blooming. Aliphatic amines are usually used as a raw material to prepare advanced curing agents or in heat-cure applications. They are also employed in room-temperature applications where appearance is less important, e.g. grouting compounds (mortars) for securing heavy machinery. In priming and / or sealing applications aliphatic amine curing agents may also be employed. Though the aesthetic appearance of the initial coating will be affected, application of another layer in subsequently may overcome the problem of amine blush.
To eliminate the formation of amine blush or amine bloom, a wide range of modified amine curing agents have been designed. Two major categories of these are are epoxy-amine adduct hardeners and a special class of adduct hardeners called Mannich-bases.
Amine bloom or amine blush not as likely to be formed in systems that have a rapid epoxy-amine cure rate. The reason for this is that the epoxy-amine rate or reaction is determined by both the reactivity of epoxy resin and the reactivity of the hardener (amine).
To reduce amine blush / amine bloom, you should avoid the use of epoxy resin components that reduce the reactivity. For example, aliphatic mono-functional reactive diluents, (often used in epoxy floor paint applications, are low in reactivity. D.E.R.* 324 epoxy resin is an example of such an epoxy. Reducing of the quantity of D.E.R. 325 epoxy resin, or complete elimination of D.E.R. 331 epoxy resin, will enhance the overall epoxy-amine reactivity.
The majority of epoxy coating systems will endure a certain degree of humidity before becoming affected. To reduce the risk of amine blush or blooming, there are several useful guidelines here:
Use of industrial dehumidifiers or heating systems (hot-air blowers) help to promote the epoxy-amine reaction rather than carbamate/carbonate formation and might be considered helpful in confined spaces. A word of caution here is that using gas-burning engines such as fork-lift trucks and so on, as well as direct-fired gas or kerosene heaters (“salamanders”) significantly increases the CO2 content in the air, (usually around 350-1500 ppm). In addition, direct-fired heaters also produce a lot of of water vapor.
This increase in water vapour is significant as most coating systems are quite sensitive to significantly elevated levels of humidity. For this reason, these systems should not be applied when moisture levels in the atmosphere are high. Usually, a relative humidity of 85% at 21 °C or 75% at 10 °C should not be exceeded for most epoxy binder systems. In small spaces, using industrial dehumidifiers and/or hot air-blowers may therefore assist in creating correct curing conditions.
Checking the temperature of the substrate is at least 3 °C (5 °C is better) above the dew point before a flooring or coating can be applied is a vital step. If the substrate surface is cool, condensate may form on the substrate or coating will can result in amine blush or amine bloom. Also, please be aware that for spray applications, greater care has to be employed because of fast solvent evaporation. This can further reduce the coating temperature, taking it down below the dew point. Finally, be aware of the influence of the temperature drop in the later afternoon or early evening in order to avoid the creation of “shadow areas.”
Before coating a mineral substrate, testing the moisture content of the surface to be painted is an essential step. When applying concrete paint to floors the residual moisture content should not exceed 4% for installing standard epoxy binder systems. When working with humid or “green” concrete, special epoxy binder systems are available. These allow for installation where there is a higher substrate moisture content.
When curing conditions are just within the limits, but it is judged that the chances of carbamate/carbonate formation are still a high risk, additional precautions can be taken. As soon as the curing agent and the resin component have been mixed together, the polymerization reaction starts. The further along the polymerization reaction has proceeded, the lower the likelihood of a reaction taking place between the amine, CO2 and moisture in the surrounding atmosphere. Mixing of the individual components thoroughly and then leaving the binder to “rest” in the mixing container before application will accomplish this objective. The pre-reaction time, which is also referred to as the “induction period“, will allow the opportunity for the free amine (typically the most reactive) to react and therefore improve the compatibility between the curing agent and the resin component. Extra care should be taken to ensure the reaction does not proceed too far and that sufficient time remains for the subsequent application of the whole batch.
You might have done your best to follow the prevention and the precautions above, but still find that there are still signs of blush or bloom in the finished job. Please don’t despair at this point as there may still be an opportunity to “rescue” the coating before more intensive repair is required.
One of the first things you can do if you spot the formation of haze, either in or on the coating, is to use heat on the affected area. In this circumstances, you should take care not to apply the heat directly to the coating. In outside areas, this measure might be more of a challenge to carry out than in smaller spaces, particularly given the limited time available.
Epoxy resin-based sealers/primers applied on surfaces such as concrete, can be subject to blushing. This is due to the moisture from the porous surface being absorbed immediately prior to curing. The epoxy primer can become hazy as a result of moisture becoming trapped. It is sometimes possible to make an repair here by immediately applying another coat of the same solvent-containing product or alternatively by saturating the coating with solvent. The newly applied layer of solvent-borne coating or the neat solvent will often soften or plasticise the coating enough to allow the trapped moisture escape and the tiny water droplets which are the cause of the the white haze to disappear.
In case a floor coating has been affected, it can sometimes still be possible to apply an additional, filled layer without having to resort to grinding, however, this additional filled layer must be applied before the first affected coating has been allowed to completely cure. This technique can work because the newly applied epoxy layer can “dissolve” the top layer of the previous coat and chemically fuse with it. Of course, it must be stated that you should make sure that the new coat will not also be susceptible to amine blush or bloom. If the coating has dried entirely, minor amine blush may be still be corrected by polishing or compounding. Another solution might be repeated washing. In some circumstances, this may bring improvement. Some people report success here when using water, whilst others recommend using a citric acid solution or dilute vinegar.
When amine blushing is severe, the epoxy floor paint will need to be sanded and refinished. Where mechanical methods are employed, the grinding of the top surface of a coating will produce a great deal of dust. Anyone working in this environment should be provided with adequate safety gear. In the most extreme cases, the coating must be removed completely before attempting re-coating. This has a major impact on the time and labour cost required to complete a painting job, and this can make the application far more expensive than initially envisaged.
Issues with amine bloom and blushing can be largely be avoided if correct planning and preparation are carried out. Please read this guide thoroughly to ensure you have taken the appropriate preliminary work and have carried out suitable humidity and temperature tests, including taking account of conditions throughout the curing period after applying epoxy floor paint.