The Ultimate Guide To Epoxy Coatings
- Epoxy coatings are used because of their outstanding chemical resistance, durability, low porosity and strong bond strength. These protective coatings are easy to field applied, brush-on, roll-on, or trowel-on coatings.
- Epoxies consist of a ‘base’ and a ‘curing’ agent. The two components are mixed in a certain ratio. A chemical reaction occurs between the two parts generating heat (exotherm) and hardening the mixture into an inert, hard ‘plastic’.
- Epoxies can yellow, chalk (or more commonly some can lose their gloss), in direct sunlight (UV). The yellowing can be a real problem. For pigmented epoxies select colors that are dark or contain a lot of yellow (such as green). Even clear epoxies can yellow and cloud up. Often epoxies are top coated with latex or urethanes that will retain their color and attractive gloss. This is particularly true if color coding or matching company colors is important.
- After the two epoxy parts are combined there is a working time (pot life) during which the epoxy can be applied or used. Generally, the pot life will be anywhere from minutes to one hour or longer. At the end of the pot life, the mixture becomes very warm (or even dangerously hot) and quickly begins to harden.
- Epoxies will harden in minutes or hours, but complete cure (hardening) will generally take several days. Most epoxies will be suitably hard within a day or so but may require more time to harden before the coating can be sanded.
- In theory, a temperature change of 18 degrees F. will double or half the pot life and cure time of an epoxy. Higher temperatures will lower the viscosity (thin) the epoxy, but also reduce the working time a person has to apply the epoxy. Spreading out the mixed epoxy instead of keeping it concentrated in a bucket or container will extend the pot life.
- Generally, epoxies can become too thick and cure too slowly to be applied at temperatures below 40 or 50 degrees F. Temperatures in the 60s, 70s, or low 80s, are best. After the epoxy has cured, it can handle temperatures well below zero degrees F.
- Most epoxies will begin to soften at about 140 degrees F, but will reharden when the temperature is reduced. For common epoxies, this temperature is the approximate upper end of working temperature range of epoxies. Special high temperature Novolac epoxies do exist, however.
- By their nature, most epoxies are hard and brittle. Additives can be added to epoxies that make them less brittle, but generally at the loss or reduction of other positive epoxy properties such as chemical resistance.
- There are special Novolac epoxy formulations that have increased chemical resistance, increased temperature resistance, and the ability to be applied underwater.
- Epoxies can be expensive, but there are ways to ‘water down’ the epoxies with less expensive solvents an/or non-solvent thinners. These cheaper, diluted epoxies do not perform as well as the more expensive, unaltered epoxies. Diluted down epoxies are especially common with ‘floor epoxies’ where pricing pressures are especially strong. To a large degree you do ‘get what you pay for’. A common non-solvent thinner is a chemical known as nonyl phenol. This chemical is sometimes used in small amounts to make epoxy mixing ratios easy whole numbers. However, cheap epoxies may contain large amounts of this inexpensive chemical. Check your epoxy’s MSDS for references to nonylphenol.
- Another clue of a cheap epoxy is if it requires hazmat shipping. Generally, the better resin systems can be shipped non-hazmat.
- Other clues of cheap epoxies include ‘induction time’ (after mixing the two components the mixture must sit for several minutes to ‘self-cook’ before being applied), and crystallization of either part A or part B if left sitting for several months (like crystalized honey, simple heating will dissolve the crystals).
- As they cure most epoxies ‘blush’. Blush is a waxy coating that forms on the surface of the curing epoxy due to moisture in the air. Because nothing sticks to the waxy coating (including paint or additional layers of epoxy) it must be washed off. Most cheap epoxies blush to some degree but some of the very best epoxies do not, in fact, some can actually be applied underwater.
- The best time to recoat epoxy is within about 48 hours after the initial coat. Because epoxies take days to reach full cure, a second coat applied shortly after the first coat will partially fuse to the first coat rather than forming a simple mechanical bond.
- Always mix the epoxies in one container then pour it into a second container and apply it from the second container. The reason is that mixing is never very good at the corners, edges, and sides of the mixing container. If you apply the epoxy from the primary mixing pail you will certainly get some of the unmixed epoxies from the bottom of the container and that epoxy will not harden. Transferring the epoxy to a second container leaves the unmixed epoxy behind, or blends it into the well mixed epoxy.
- The difference between polyester (fiberglass) resins (commonly used in fiberglass boats) and epoxy resins: Polyester resins are much less expensive, have very strong fumes, are more porous than epoxy resins, and only sticks really well to itself. For anti-blister marine barrier coats, and bonding to wood, steel, etc. use epoxy resin not polyester resin. Generally, epoxies (which are often solvent-free) can be applied to foam products whereas the polyester resins will dissolve these products.
- End users can thicken epoxy with many things, Tiny glass spheres, known as microspheres or micro-balloons are commonly used. Besides thickening, their crushable nature makes sanding the hardened epoxy easier. On the downside, they can work like tiny ball bearings, resulting in sagging and slumping. Another thickener is fumed silica (a common brand name is Cabosil ™ which looks like fake snow. About 2 parts fumed silica with one part epoxy will produce a mixture similar in texture and thickness to petroleum jelly. Micro-spheres and fumed silica can be combined together.
- While epoxy floors are very common, for serious and demanding applications the epoxy is either mixed with or applied under and above, quartz (sand) or aluminum oxide grains. Either way, the result is really a quartz or aluminum oxide floor, held in place with the epoxy. The quartz, and even better the aluminum oxide, is much more durable and wear resistant than the epoxy alone.
- How thick should your epoxy coating be? Thicker is not necessarily better. The paint on your office walls is probably 2-4 mils thick (1000 mils = 1 inch). Ten mils are considered a fairly thick industrial coating. A gallon of epoxy applied at 10 mils will cover 160 square feet. That same coating, applied 1/4 inch thick, will only cover 6.5 square feet. To be price competitive with the 10 mil coating on a cost per square foot basis, the quarter inch thick coating would have to be very inexpensive. A primary way to reduce cost is to use low quality resins and lots of cheap fillers. As a result, the thicker coating may be inferior to the thinner, higher quality coating.
- Adhesion of underwater applied epoxies: Underwater epoxies generally have good to excellent adhesion to most submerged surfaces, (i.e. emergency boat hull repair) however, steel surfaces in saltwater environments can be a problem. Such surfaces are often protected by a cathodic protection system. These systems use electrical current to suppress corrosion. Dissimilar metals in saltwater also form tiny electrical cells. Because epoxy bonding is due to the molecular attraction of charged particles, existing electrical charges, known or unknown, can interfere or disrupt epoxy bonding. It is best to test underwater coatings for possible cathodic adhesion problems if used in marine settings on steel surfaces.
- Epoxies and other paints/coatings should not be applied directly to galvanized surfaces. Galvanization is itself a protective coating, one that works by forming its own protective layer. Epoxies applied to galvanized surfaces can soon peel off. If galvanized surfaces must be coated, be sure to use an approved primer. For reasons not known to the author, aluminum is also another metallic surface that epoxies sometimes have a difficult time getting a good bond to.
- Fisheyes are areas on a painted surface where the coating literally pulls away from the substrate leaving a coating less void or fisheye. Often fisheyes are caused by surface contaminants such as a bit of silicon, wax, or oil. I have also seen them on clean plywood where epoxies paints have been used as sealers and the problem might be due to uneven saturation (soaking-in) of the epoxy into the wood. Surface tension plays a big part in fisheyes. There are some additives that can be mixed into the epoxy that will reduce surface tension. Likewise, on wood, applying several coats of solvent thinned epoxy, instead of one coat of unthinned epoxy, seems to work well. Applying a thick coat of epoxy over a contaminated fisheye surface will bury the fisheye but expect the coating to peel away in the future. As a rule of thumb, always suspect some sort of surface contamination as the primary cause of fisheyes. Pinholes are similar but caused by expanding air bubbles under the still soft epoxy. Coatings can fail for lots of other reasons.
- Adding a bit of solvent to a solvent based or solvent-free epoxy is something that most manufacturers would not officially approve of and something that might not work with all epoxies. However, it can be done (unofficially) with the epoxies I deal with. Adding solvent to these epoxies will: 1) thin them out; 2) increase pot life; 3) allows them to flow off the brush/roller a bit more smoothly; and 4) perhaps allows them to ‘soak-in’, penetrate, or may be softened, the substrate just a little bit. Not change is visible in the epoxy unless 10% or greater solvent is added. With that amount of solvent, the epoxies no longer cure with a glossy finish.
- It is best to use epoxies with a mix ratio close to 1 to 1 as opposed to something 4-1, 5-1, etc. because errors in the mix ratios can be more pronounced with the latter. That said, no matter what the mix ratio is, some epoxies are more forgiving of mix ratio errors than others. One ‘trick’ of epoxy vendors with odd or very sensitive mix ratios is to sell calibrated pumps that dispense the epoxy components in exact amounts.
Epoxies made with Bis F, a Bis F and novolac mixture, or Novolac resin exhibit greatly improved chemical and heat resistance compared to the much more common Bis A epoxies. Probably 98% of all epoxies are ‘regular’ Bis-A epoxies. Assume Bis-A epoxy unless specifically told otherwise. Novolac epoxies are more expensive than regular epoxies. They exhibit higher ‘heat distortion temperatures’, higher ‘T sub G temperatures’ (both of these are measures of when the epoxy begins to soften with heat). The values for these measurements vary slightly with the different resins, and by whether the vendor reports conservative values or optimistic numbers.
Generally, Bis-A epoxies will begin to soften in the 120-160 degree F range. Novolac epoxies initially raise this value by about 25 degrees F. More important is what happens above this temperature. All epoxies will reharden when the elevated temperatures fall below this transition temperature. However, Bis Novolac epoxies will continue to cure when exposed to temperatures of about 150 degrees F for a few hours.
After this ‘additional curing’ they generally can withstand about 300 degrees F (dry environment) without problems. An exception to this is our non-hazmat novolac epoxy (FC 2100 N). The non-hazmat curing agents used greatly reduce the temperature resistance (but not the chemical resistance). For ‘true’ novolac temperature resistance use our Nova Clear hazmat novolac epoxy.
Chemical Resistance: A good quality Bis-A epoxy will handle 70% sulfuric acid. A novolac epoxy will handle 97 or 98% sulfuric acid.
ECO-POLYMER is a new generation of hybrid 100% solids NOVOLAC epoxy coating and repair system with optional rubber, ceramic and Kevlar™ additives, providing outstanding impact resistance, great wear, excellent compressive and tensile strength and elongation.
Eco-Polymer may be applied to damp or wet surfaces and have proven to have outstanding resistant to gasoline, jet fuel, diesel fuel and other petroleum products. Combined with bonding strength greater than 3000 psi, this NOVOLAC epoxy has proven to be more than four times more effective against salts, alkalis, sewage and dilute mineral acids than traditional epoxies. ECO-POLYMER has proven ideal for protecting concrete and steel in aqueous service in water/waste management, wet wells, and manholes.
Today with new curing agents, this hybrid NOVOLAC resin combine for ease of application, by roller, brush or spray, and is safe for the applicator and the environment. These used friendly products are formulated without solvents or VOCs and are basically odorless, so they may be applied in occupied areas.
Getting Your Money’s Worth
The reason for coating and repairing an asset is to extend its service life and provide protection from erosion and corrosion damages of chemicals and wear from abrasion or impingement.
The only way to reduce the price of a coating product is to use inferior resins and or fillers like solvents.
The purchase price of a coating or repair material by its self is a very small part of the overall cost of most projects.
Though, purchasing what may appear to be a bargain material at the time, that later proves to cause health problems or fails to provide the proper protection to the asset and shortens is usable life cycle, is the most costly of errors.
The Problem With Solvents
Most States and the EPA have regulations regarding the acceptable levels of these carcinogenic agents allowed in products due to health and environmental concerns. Not only are they a health hazardous substance, they are one of the primary causes of coating failures. As the solvents evaporate into the air, and water the coating becomes thinner and holes are created in coating’s structure, causing permeability problems. This process continues long after the original curing process. The results are a failing coating system that will allow the penetration of harmful chemicals to undermine the coating and damage the structural base metal or concrete.