What is the difference between Thermosetting and Thermoplastic Coatings?
Thermoset coatings are those that are applied in a liquid state and cure, in place, by means of a chemical reaction thereby becoming insoluble in solvents. The chemical reaction, or crosslinking, converts lower molecular weight materials (small molecules) into high molecular weight materials (large molecules or a network of large molecules).
Typical examples of thermoset materials include epoxy materials (amine, polyamide, or polysulfide cured), inorganic silicates (such a IOZ), polyurethane materials (two package, moisture-cured, or elastomeric), alkyds/drying oils, silicones, and polyester/vinyl esters. These are the types of materials that have been discussed thus far, and represent the high performance end of the industrial coating types.
Thermoplastic coatings are coatings that remain soluble in solvent or a blend of solvents after drying (cure). Usually a single package material that dries and cures by solvent evaporation from a solution, or the evaporation of water from an emulsion coating, are thermoplastic coatings. Examples include acrylics, vinyls, chlorinated rubbers, coal tar and asphalt enamels, hydrocarbon and natural resins.
Acrylic-based coatings are generally thin film (1-3 mils dft) products that are principally used as finish coats (topcoats). Similar to polyurethanes, acrylics are much more chalk-resistant in exterior environments than are epoxies. Industrial acrylic coatings have traditionally been solvent-based products, but this has been changing recently and many adequately performing water-based acrylics have become available. The main advantage of acrylics is their ability to maintain their original appearance in mild weathering environments for longer periods than many other coating types. Acrylics also have the added advantage, when compared to vinyls, of being suitable for moderately hot surfaces such as motor housings, process piping and tankage up to 300oF or more, depending on the specific product.
Many water base decorative house paints are based on acrylic emulsions that typically maintain appearance and durability better than less expensive polyvinylacetate (PVA) type emulsions. Certain industrial grade block fillers and masonry type products are also formulated with acrylic resin types.
What are the advantages of Acrylic-based coatings?
Typical advantages of acrylics are excellent weathering resistance (color and gloss retention). Acrylics are also flexible, resist yellowing at higher temperatures, and are generally single package formulations. Typical disadvantages of acrylics are poor solvent resistance, soften at higher temperatures, and have narrow application windows.
Another major coating type is the vinyl coating; previously a dominant industrial coating. High performance vinyls could only be formulated in solutions that were low in coatingcoatings solids and had high VOC (volatile organic content) levels. The rise of regulation mandating the reduction of VOC emissions has led to the shift from vinyl coatings to the epoxy type of coating that can be formulated in much lower VOC formulations. About the only place you commonly see vinyls applied anymore is when someone uses liquid paper. But with the advent of word processing programs, liquid paper use is also declining.
What are Vinyl Coatings used for?
Vinyls are typically used as finish coats (topcoats) because of good overall chemical resistance and flexibility. Bridge structures are a major area of usage where vinyls are well suited to withstand moisture, road salts and widely varying temperature extremes.
What are the advantages of Vinyl Coatings?
Advantages of vinyl coatings include single package formulations. Additionally, vinyls dry very quickly. Most commercial vinyl resins are solids and require dispersion in polar solvents such as MEK or MIBK. These common solvents evaporate extremely fast, which is the reason that vinyl paints are very fast drying coatings. The Carboline equivalent is identified by the trade name Polyclad. Vinyl coatings are generally very good adhering coatings. The vinyl resins are modified to improve adhesion characteristics by incorporating plasticizing compounds that help the flexibility characteristics. Vinyls have excellent resistance to water, acid and alkali; and poor resistance to strong solvents, particularly the ketone solvents. Good color retention is characteristic of low solids vinyls, but generally decreases in higher build commercial products. Weathering properties are fair.
What are the disadvantages of Vinyl Coatings?
Several disadvantages are inherent in vinyl coatings. Vinyls have poor heat resistance that results in yellowing at temperatures over the 160 to 180oF range. The fast drying characteristics on outdoor structures applied under windy, high temperature conditions can result in unsightly lap marks that are aesthetically displeasing.
Also, due to the solubility characteristics of most vinyl resins at working viscosity, the finish coatings are lower in solids than many other generic types. When topcoating other generic coating types, the vinyl solvents can lift many other coating types, so test patches are generally recommended before the coating specification is finalized.
Chlorinated rubber coatings are another widely known group that are principally used by foreign specifiers, and have a long history of use in other parts of the world, particularly Europe. They are particularly well suited to moist environments and offer outstanding barrier properties in wet environments. At the same time, they are reasonably flexible (although not quite as good as vinyls) and also have fair film build properties (3-5 mils dft). Unfortunately, the VOC levels of chlorinated rubbers are high, so they are being generally supplanted by epoxy in many applications.
What is Chlorinated rubber used for?
Prime areas of use include many marine related industries such as ships, barges, offshore structures, tidal structures, and swimming pools, as well as areas such as sewage and treatment. These products are also used to protect concrete and masonry in wet environments.
What are the advantages and disadvantages of Chlorinated rubber?
Advantages include single package formula, fast drying, water resistant, chemical resistant, and flexible. Disadvantages include low film build application, high VOC content, limited availability and cost (expensive), difficult application characteristics, and hazardous ingredients (carbon tetrachloride).
Tar and Asphalt Enamels
Tar and asphalt enamels are thermoplastic products made from coal processing-tars and oil processing-asphalts. These materials are generally solid at room temperature, but become soft or fluid at higher temperatures. They are capable of forming barrier coatings, but are much less costly than epoxy materials.
The advantages and disadvantages of Tar and Asphalt Enamels
Advantages include single package formulations, minimal surface preparation needed, resistant to acids, alkalies, and water. Disadvantages include poor solvent resistance, softening at elevated temperatures, and application complexity (either hot melt or low solids application).
Hydrocarbon and Natural Resin
Hydrocarbon and natural resin coatings are formulated from a range of any number of resinous materials that have been found to form a film upon drying. Many of these resinous materials are not even characterized definitively, but simply have been found in some underground deposit, found to form a film, and are being used. These types of materials are infrequently used in industrial coatings, and most often as primer resinous components. Their cost is generally much less than epoxy materials.
The advantages and disadvantages of Hydrocarbon Resin Coatings
Advantages include single package formulations, easy application, and fast drying. Disadvantages include solvent sensitivity, softening at elevated temperatures, thin film application, and extremely high VOC levels.
The information presented showing the advantages and disadvantages of different generic types of coatings will provide perspective for coatings selection, resulting in long-term effective corrosion protection. Hopefully, the comparisons with epoxy materials will emphasize the strength of epoxy materials, as well as indicate alternatives where epoxy materials have characteristic weakness.