Industrial paints and coatings are usually categorized by the major resin contained in the product. We categorize paints and coatings so we will have some idea of what to expect when a coating is specified, applied, or repaired. In general, the predominant physical characteristics and chemical resistance of a coating are determined by that resin choice. The other ingredients, or components, merely enhance this capability or perform other specialized functions.
In the past twenty years, epoxy coatings have emerged as the dominant generic type used in industrial paints and coatings. This is a change from the previously dominant types such as vinyls, and alkyds. In this presentation, we will first consider epoxy coatings, including the common variations. Then we will discuss the other generic types of coatings in relation to the functions that they perform to add to the properties obtainable from epoxies. In presenting each generic group, we will make comparisons to epoxies with the intent of creating a mental “ladder” that emphasizes the properties of the various coating types. These comparisons will be accomplished by discussing the advantages and disadvantages of each category. Hopefully, this information will help promote knowledge that leads to intelligent choices, which maximize corrosion protection into long term economical performance.
What is the difference between Thermoplastic and Thermosetting?
Coatings are divided into two classifications; thermoplastic or thermosetting (read more). Epoxies are thermosetting. Thermoset coatings are materials that cannot be returned to their original state by contact with thinner, heat or other solvents. The cure process chemically converts a thermosetting coating. The cure process may be performed by the blending of two chemically active components, by ambient air oxidation of the resins, by ambient moisture conversion of the resins, by the application of heat at elevated temperatures, or any combination of the above. Alkyds and urethanes are other examples of thermosetting coatings. Thermoplastic coatings can be dissolved back into the liquid state by their original thinner or other selected solvents. Thermoplastic coatings are not chemically cured, but only physically dry through loss of volatile components. Examples of thermoplastic coatings are acrylics and vinyls.
What are VOC’s Coating products?
In the last several years, due to environmental and safety legislation, much greater emphasis has been focused on low VOC coating products. VOC is by definition the “Volatile Organic Content”, or volatile solvents that evaporate from coatings and are released into the surrounding environment. Since VOCs are easily monitored, a major attempt to reduce these volatile products is in place. Carboline Company has been an industry leader in this area while attempting to maintain outstanding qualiy and user friendly behavior of its products. We feel we have been successful in this area and will continue to be corporately responsible in providing products compatible with environmental concerns and customer needs.
The 6 types of Epoxy Coating Epoxies
Thermosetting coatings are generally characterized by being two package, or two component, materials that must be mixed together to initiate a chemical reaction. One of the most widely used thermosetting type coating (and in heavy-duty areas the first choice) is the epoxy type. Presently, four major product variations of epoxies contribute their unique properties to the marketplace. They are epoxy amines, epoxy polyamides, epoxy coal tar, and aluminum epoxy mastic.
Epoxy amines are generally the most chemically and physically resistant members of this family. Chemical, solvent, and water-resistant are typical of epoxy amine coatings. Additionally, they are characteristically medium to high build capable (4-8 mils/coat) and high in solids. Epoxy amines are generally used in tank lining or severe service areas. Air dry epoxy phenolics and, to some extent, the baking finishes also utilize an amine curing agent. Phenoline 187 is an amine cure epoxy used as a tank lining for fuels, freshwater and saltwater service.
The amine-curing component in the liquid state is extremely moisture sensitive, which necessitates high surface cleanliness and careful handling to properly apply these products. They are generally categorized by an odd mixing ratio of 2:1, 10:1 or 20:1. Mixing of these products becomes much more critical and small amounts of catalyst not mixed can result in poorly cured finishes. Potlife is short, the amines are corrosive and irritants, chalking and color change frequently occur, and the amine can float to the surface in cool, moist conditions and form a surface blush that must be removed before recoating.
Epoxy polyamides utilize a polyamide curing agent that is simply a pre-reaction of basic amine-curing agents that result in lesser chemical and physical resistance, but renders the curative more moisture tolerant. This moderation of moisture sensitivity leads to easier application characteristics and more versatile physical properties of the cured film. The epoxy polyamides are more flexible than epoxy amines, retain color better, have a longer pot life, and are more moisture tolerant. Adhesion is very good with epoxy polyamide coatings. Typical of these products are the simple 1:1 mix ratios.
Disadvantages of epoxy polyamides with respect to epoxy amine coatings are lesser solvent and chemical resistance, and lower solids capability. Epoxy polyamides are subject to chalking as are the epoxy amines.
Areas of use for epoxy polyamides include structural steel, tank exteriors, process piping and equipment exteriors. Industries likely to utilize these coatings range from light manufacturing to the heaviest industrial environments.
Another group of products, based on relatively new coatings technology, are categorized as epoxy cycloaliphatic amines. This smaller, but rapidly growing group of coatings has characteristics that generally are hybrids between amine and polyamide products.
Corrosion and chemical resistance generally is better than polyamide products but not quite as good as some amine materials. These products have been developed to meet high solids-low volatile organic content, yet amazingly are easy to use, moisture tolerant and aesthetically pleasing products well received in the marketplace in a relatively short time.
Aluminum Epoxy Mastic
Another widely used group of epoxy material is the aluminum epoxy mastic. This type product is a relatively recent material with Carboline being the forerunner in introducing a commercial product that has been widely used as a result of greater emphasis on maintenance coating applications.
The major advantage of this product is the ability to perform well over rusted steel surfaces and many existing coatings without the need for extensive surface preparation. Both epoxy amines and epoxy polyamides generally require an abrasive blast cleaned surface. Aluminum epoxy mastic is also high solids and can be in turn topcoated with most generic types of topcoats. Solids levels are high and VOC levels are low. Aluminum pigmentation, particularly leafing-aluminum, increases the corrosion protection capability significantly. The aluminum epoxy mastics are typically easily brushed and rolled, in addition to the typical spray application.
The primary disadvantage of aluminum epoxy mastic coating is the relatively slow curing to topcoat or put into actual service. Airborne overspray remains wet and liquid much longer and leads to overspray problems more severe than that found for coatings that dry fall.
Epoxy Coal Tar
Epoxy coal tar is a hybrid form of the epoxy type of coating, but is so widely used that it deserves special discussion. By incorporating coal tar or other bitumen by-product into standard epoxy, they develop the capability of becoming outstanding moisture barriers, as well as improved resistance to hydrogen sulfide, acids and alkalis. Epoxy coal tar coatings also are high build products (6-10 mils per coat and higher) and low in cost. Industries such as marine, offshore exploration, waste treatment, and pulp and paper utilize these coatings.
Disadvantages are also introduced into the coal tar epoxy with the coal tar. Because the bitumen by-products such as coal tar are black, or very dark in color, color choices are extremely limited in the epoxy coal tar products. Solvent resistance is low, topcoat ability is reduced, and the coat tar itself is an irritant to many individuals.
Epoxy Penetrating Sealers
Epoxy penetrating sealers are a combination of much of the epoxy technology discussed above. High resin levels, incorporating much less pigment and other additives result in a coating with fluid penetrating properties for the epoxy penetrating sealer. The surface tension of the sealer is low which also allows for good wetting of the surface, even when marginally prepared. A wide variety of previously applied coatings can be overcoated. The curing stress on previously applied coatings is reduced with the selection of the appropriate epoxy resins. Generally, many types of topcoats can be applied over the sealers.
Disadvantages include low film build limitations, “puddling” must be avoided on horizontal surfaces, and the sealers must be topcoated as UV resistances is poor.
Types of Inorganic Zincs
Inorganic zincs are discussed in greater detail in other presentations, but a few details regarding this popular product are appropriate. Zinc-rich inorganic coatings provide exceptional corrosion protection over an extended span of years, performance that can not be equaled in epoxy coatings. In general, inorganic zincs can offer the same level of protection as standard hot-dipped galvanizing. The most commonly used inorganic zinc is the solvent-based, self-curing alkyl silicate with metallic zinc in the 75-90% zinc-in-the-dry-film weight level. These products offer exceptional abrasion and handling resistance. Because they are fast drying, inorganic zincs are well suited for use by fabricators. Disadvantages include poor resistance to acids and bases, the requirement of mechanically cleaned (abrasive blast cleaned) surface preparation, and topcoat bubbling.
Zinc-rich inorganic coatings are formulated with zinc levels of 75% plus zinc (metal by weight) in the dry film. Both solvent-based and water-based inorganic zinc-rich are available. The solvent-based IOZ’s are typically self-cured ethyl silicate based. Water-based IOZ’s can be any of a number of chemical types including post-cured, self-cured, sodium, lithium, ammonium and potassium silicates.
Solvent-based Zinc-rich Inorganic
Solvent-based inorganic zinc provides excellent corrosion protection, is fast drying, develops excellent adhesion and provides a mechanical bond with topcoats. Cure can be achieved at temperatures as low as 0oF, application can be with airless or conventional spray, there is a wide tolerance of acceptable film thickness, and the cure is promoted by the presence of moisture.
Disadvantages include the tendency to dry spray in hot and/or windy conditions, the development of a porous film easily subject to oily contamination, sensitivity to both acid and alkali, and higher VOC levels than some non-zinc rich primers.
Water-based Zinc-rich Inorganic
Water-based inorganic zinc provides the corrosion protection of all inorganic zinc coatings with virtually zero VOC levels. Dry and cure is rapid, adhesion is excellent, substrate is excellent, mechanical adhesion of topcoats is good, and cleanup is with water.
Disadvantages are those found with the solvent-based IOZ’s (with the exception of high VOC) and generally difficult spray application. The zinc particle sizes are larger so agglomeration and tip clogging during spray application can easily develop.
Organic Zinc-Rich Primers
While inorganic zinc-rich primers offer the highest durability by all experience, there are many situations where the longest durability is not necessary. Zinc metal can also be incorporated into any number of resin systems to become what is called anorganic zinc rich primer. The benefits of the zinc metal in providing cathodic protection are still available, although reduced, and the coatings are more user-friendly and lower in cost.
Epoxy zinc rich is the most popular organic zinc rich at this time, although vinyl, chlorinated rubber, alkyd, urethane, and silicone zinc riches have all been offered to the industrial coatings market at various times.
The organic zinc rich primers are more tolerant of surface preparation. While IOZ’s require abrasive blast cleaned surfaces to a commercial or near-white quality, organic zinc rich coatings can often be applied to power and hand tool cleaned surfaces. The application difficulty of using organic zinc rich coatings is much less than IOZ. Zinc powder does not settle as readily due to the higher viscosity of the organic resin. Spray application is much easier, with less dry spray and tip clogging. It is easier to brush touch-up organic zinc because the liquid coating is much smoother in its flow out.
Cathodic protection is reduced with organic zinc rich coatings, but the resulting protection is frequently adequate. The resin system of an organic zinc rich coating will degrade in outdoor exposure much sooner than inorganic zinc-rich, due to the exceptional stability of the silicate structure. The degradation of the organic zinc rich coatings follows that for the resin with which it is formulated.
The different types of Polyurethanes Coatings
Polyurethanes are one of the most popular coating types currently in use for corrosion protection. Epoxy coatings are inherently sensitive to ultraviolet light attack, so the epoxy coatings will chalk in exterior exposure. To provide better exterior chalk resistance, the polyurethane class of coatings has developed. Polyurethanes are inherently much more chalk resistant than epoxy and have become the finish coat of choice in applications where color and gloss retention are valued. Polyurethanes generally offer the chemist and formulator the extreme versatility of products ranging from high gloss thin film (1.5 to 2.0 mils dft) chemical resistant topcoats to heavy duty thick waterproofing membranes that are 40 to 125 mils thick and extremely flexible. This latitude of product characteristics helps explain the widespread use of the materials by basically all industrial categories in addition to exterior topcoat service.
Aliphatic Polyurethane Finishes
Advantages of polyurethanes include excellent weathering performance, high gloss obtainable, good adhesion, high abrasion resistance, and flexibility. Disadvantages of aliphatic polyurethane finishes include the extreme moisture sensitivity of the liquid paint and uncured coating film. Water sensitivity can cause problems during application such as foam, bubbles, and craters in the cured film. Film build of polyurethane gloss finish coatings is typically low, that is the 2 to 3 mil dft range. Polyurethanes are expensive compared to epoxies, temperature sensitive, and allergenic sensitizers for many applicators.
Aromatic Polyurethane Finishes
Aromatic polyurethane finishes are very high performance lining materials with good chemical resistance, solvent resistance, and abrasion resistance. Additionally, they cure well at low temperatures. Disadvantages include rapid discoloration, chalk sensitivity, health concerns, and moisture sensitivity during application. These products are alternatives to epoxy tank linings, but have not generally been able to displace epoxy tank linings.
Polyurethane elastomers are another class of polyurethane coatings. These are thick film materials with particular suitability over concrete substrates. Advantages include high elongation, good chemical resistance, water resistance, high build properties, abrasion and impact resistance. Disadvantages include a moisture sensitivity shared with the other polyurethane materials, health concerns, high film thickness required, and marginal adhesion properties.
One of the most widely used coating types are alkyds. These coatings are more widely used in the architectural and product finishes market, rather than the industrial coatings market. They were the dominant industrial coatings type in the first half of the 20th century, but have been supplanted by epoxies most recently. The major reason for their continued usage in the industrial market at all, relies heavily on the fact that alkyds are low cost coating materials in terms of both material and application labor required to install.
Alkyds are commonly referred to as oil based paints and the oil type is frequently mentioned in the product description. Examples are linseed, soya, tall oil, castor, etc. Natural or synthetic polyols can be used to manufacture alkyds. Chemically an alkyd coating is a polyester material that has been chemically converted through air dry oxidation of the drying oils present. Silicone polyols can be used to prepare silicone alkyds with enhanced durability performance. Epoxy resins, as well, may be incorporated into an alkyd resin to enhance the durability of the alkyd, with the resulting alkyd coating being described as an epoxy ester coating. Even urethane resins have been used to modify alkyd resins and the resulting urethane alkyd being called an oil modified urethane or polyurethane. In many cases the inhibitive pigment is also described and includes zinc chromate, red lead, and red iron oxide, as well as many proprietary nonleaded and chromate free pigments suitable for the environmentally conscious marketplace.
If a single word were used to describe alkyds, it would be versatile. These products may be formulated to vary from exceptionally fast drying primers suitable for fabrication shop usage, or as slow drying, excellent wetting, maintenance type coatings for stationary structures. Alkyds are relatively inexpensive. Because they are low film thickness materials (1-3 mils typical dft), alkyds can normally be applied by brush, roller or spray equipment easily and quickly. Adhesion is generally good, alkyds can be applied over most other alkyds, and alkyds are single package materials. Alkyds are good product choices for mild environments requiring aesthetically pleasing economical coatings.
Primary disadvantages of alkyds are poor chemical resistance and multiple coat requirements to achieve any degree of protection. These products also tend to crack and embrittle as they progress through their life cycle, exposing the substrate to undercutting and sub-film corrosion. This occurs because alkyd coatings cure by ambient air drying chemical conversion. This curing slows as the coating ages, but does not stop. Eventually, alkyd type coatings reach a stage of overcure with the embrittlement and cracking associated with overcuring. Application of a heavy thickness of alkyd may require excessive curing times due to relatively slow evaporating thinners such as xylol or mineral spirits that are commonly contained in these products. In this case, “more is better” is not true. Additionally alkyds have poor chemical resistance with sensitivity to both acid and especially to alkali. Alkyds also chalk in exterior service.
Drying oils are one component of alkyd coatings, but can be formulated into paint and applied without the conversion to alkyd. A number of drying oils are commonly used in alkyds and include linseed oil, soybean oil, safflower oil, oiticica oil, tall oil, tung oil, castor oil, and fish oil. These are usually used in combination to obtain the desired drying properties.
The advantages of drying oils include high tolerance of surface conditions, good adhesion, application by brush or roller, single package, inexpensive and wide application latitude. Disadvantages include poor chemical and solvent resistance, chalking tendency, long dry/cure times, and low film build application.
Silicones are a second exception to the typical “two part” rule for thermosetting coatings. They are converted into an effective material by heat curing, and this is also their primary area of usage. Silicone resins are very heat resistant and provide a degree of high temperature service not obtainable with epoxy coatings. The ideal coating system for much high temperature service is inorganic zinc primer followed by silicone topcoat. Color retention is also good with silicone coatings.
Disadvantages include the high cost of silicone products, the requirement for heat to develop full physical properties and the limited compatibility of silicone coatings with other coating types.
Silicone resin is also incorporated into many other generic types to raise effective temperatures of service, and are generally thought to improve weathering and UV resistance. Silicone alkyds are prime examples of this type of combination.
Polyesters and Vinyl Esters
Polyesters and vinyl esters are chemically cured (peroxide curative) coatings with a high degree of chemical resistance particularly to acid conditions which epoxy coatings can not typically handle. Both can be formulated into high solids, high build films with excellent mineral acid resistance, fair oxidation resistance, and physically strong, abrasion resistant films. Disadvantages include limited solvent and caustic resistance, with the polyesters being worse than the vinyl esters. Both polyesters (sometimes called unsaturated polyesters) and vinyl esters contain styrene, significantly shrink during cure, and develop marginal adhesion. Polyesters also tend to be brittle.
Polyisobutylene coatings based on resins like Opponal by BASF are non-curing coating and can be troweled, sprayed or patched. These coatings can be top coated for UV-protection and non-tackiness by for instance an amorphous foil or 1-component acrylic without stress generation. This last coating is also referred to as an viscoelastic coating. Read more