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Chemical Resistant Concrete Coating[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Chemical Resistant Concrete Coating
- From: GSKWY(--nospam--at)aol.com
- Date: Mon, 3 May 2004 13:28:23 EDT
A while back, someone asked about coating concrete for chemical resistance. Here is some basic information.
In general, when you are looking for these coatings, you will be looking at speciality products. Your friendly Sika or Masterbuilders rep is probably not going to be much help.
Concrete Coatings for Chemical Resistance
Exposure to aggressive chemicals can cause rapid deterioration of concrete floors. If left unprotected, damaged floors can interfere with facility operation and create safety hazards for workers; chemicals that leach through the concrete can contaminate the surrounding soils.
A variety of different types of coatings are used on floors that require chemical resistance. These include epoxies, epoxy novolacs, urethanes, polyureas, methyl methacrylates, polyesters, and vinyl esters. Within each category, different formulations have been developed for enhanced resistance to specific chemicals or application under specific conditions. The coatings are typically thermosetting polymer resins that bond to the concrete surface. Most of them can be applied as a thin film or as an aggregate-filled overlays up to 3/8 in. thick. Overlays can also be reinforced with fiberglass mesh to help them bridge small cracks.
Epoxies are the most commonly used coating when chemical resistance is required. They adhere well to concrete and have extremely low shrinkage, which prevents the buildup of stresses that can cause delamination. They are generally two-component systems consisting of a resin and a curing agent. Although there are a limited number of suitable resins, there are hundreds of curing agents. Epoxies can thus be formulated with a wide range of physical properties and a wide range of chemical resistance. Epoxies typically have excellent abrasion resistance and good resistance to alkalis, bleach, moderate organic acids, and some solvents. They are less expensive than most other coatings and many are moisture tolerant. They can form high-build films and have excellent mechanical properties (tensile, compressive, and flexural strengths.) They can also be formulated as 100% solids, which makes them very useful in application where VOC emissions are a problem.
Epoxies are not resistant to ultraviolet light (sunlight), however, and they have a limited service temperature range; many cured epoxies soften at temperatures above about 150° F. Their cure rate is affected by temperature; most products should not be applied at temperatures below 40° F.
Epoxies used for general-purpose floor coatings are based on Bisphenol A resin, epoxies used for more severe chemical exposure or higher temperatures are based on Bisphenol F (Novolac) resin. Epoxy novolacs are similar to conventional epoxies but the polymer chains are more highly cross-linked. This makes them more rigid and chemical resistant, and allows them to withstand higher service temperatures. Epoxy novolacs have excellent resistance to alkalis, acids, bleach, and many solvents. Because of the higher cross-linking, however, they are typically more viscous than conventional epoxies, which can make them more difficult to apply.
Urethanes and Polyureas
Urethanes are available in a wide range of formulations, ranging from thin film to high build coatings. They have excellent abrasion resistance, relatively fast cure times and they can be used over a wide range of service temperatures. They are resistant to alkalies, moderate strength acids, some solvents, and bleach.
There are two general types of urethanes: two-component (chemical cure) and one-component (moisture cure.) Two-component urethanes cure by chemical reaction with a curing agent or catalyst. One-component urethanes cure by reacting with moisture in the air. The two-component systems generally provide better performance, but one-component urethanes are widely used because of their convenience. Both the one- and two-component coatings perform best when placed on dry concrete. Moisture in the concrete inhibits the cure of two-component coatings and can cause blistering of one-component coatings.
Urethanes typically cost more than other types of coatings and most urethanes contain solvents; some may not be VOC compliant. Urethanes are often used as a topcoat over an epoxy system; this reduces application and VOC problems, as well as costs. Epoxy-urethane hybrids have also been developed. Urethanes are classified as either aliphatic or aromatic, depending on the chemistry of the basic urethane raw material. Aliphatic urethanes are UV resistant; aromatics are not.
Polyureas are similar to polyurethanes but are based on slightly different resins. The resins used for polyureas are extremely reactive and do not require a catalyst. They cure rapidly and cure well even at cold temperatures; they are also somewhat more tolerant of moist substrates than polyurethanes. The curing time may be too fast for certain applications, however, and the abrasion resistance is typically not as good as a polyurethane. Many products marketed as polyurethanes are actually a polyurethane/polyurea blend.
Methyl methacrylates (MMAs) are fast curing overlays that have moderate resistance to solvents abd are resistant to weak acids and alkalis. They are two-component systems in which the methacrylate resin is mixed with a solid powder initiator. Because the curing is inhibited by oxygen, a paraffin wax is included in the resin. The paraffin rises to the surface of the coating, forming a protective barrier. An aggregate filler should be used to avoid entrapping air within the overlay.
At room temperature, a typical MMA has a working life of 10 to 15 minutes and reaches full cure in 1 to 2 hours. The cure rate can be maintained at low temperatures by adding more catalyst; MMAs have been placed at temperatures as low as -20° F. MMAs must be applied to dry concrete because moisture interferes with bonding. MMAs have excellent intercoat adhesion, however, and can be applied in several coats without delaminating. MMAs are resistant to ultraviolet light, so can be used for exterior applications; their maximum service temperature is 140° F.
Polyesters and vinyl esters
Polyesters and vinyl esters are resistant to a variety of chemicals, including acids, alkalis, and solvents; they also resist impact well and can withstand high temperatures. Vinyl esters are resistant to all but the strongest acids and alkalis. Polyesters are two-component systems in which a prepolymer resin dissolved in styrene is mixed with a peroxide catalyst. Vinyl ester resins are a type of polyester resin, where the prepolymers are formed by reaction of epoxy resin with acrylic or methacrylic acid.
Polyesters and vinyl esters are typically only used when strong chemical resistance is required, as they have more critical application requirements than other coatings. They have a high rate of shrinkage and a high coefficient of expansion, which can lead to cracking or debonding. To offset these properties, they are often mixed with sand or other mineral fillers and reinforced with fiberglass mesh. They typically cannot be applied unless temperatures are above 50F and the concrete is dry. The surface of the concrete must also be treated to neutralize its alkalinity. Curing times depend on the temperature and the amount of catalyst, but the slab can usually be opened to foot traffic in a few hours; full cure can take up to seven days.
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