PROBLEM SOLVING FORUM How Moisture and Humidity Affect Thin Film Urethanes Answer to be cleaned as frequently as begloss finish to reduce project cost. P. Kronborg, fore. This job is usually left to naUnfortunately, the curing agents of Hempel, ture. the chemically curing urethanes do Lyngby, Denmark: Therefore, there is no reason to not only react with the base compoChemically curing polyurethane nents of the coating material; but use a high gloss topcoat. The coatings are used for a variety of they also have a tendency to react topcoat can just as well be a semipurposes. The better known uses much more willingly with water gloss, the condition a high gloss are related to their excellent would reach after contagloss and color retention, as mination from dirt and well as their good chemical debris in the atmosuestion resistance. Typically, they phere. Color, however, is What are the effects of moisture and humidity are used on automobiles, still important. A semion newly applied thin film, chemically pleasure boats, and passengloss topcoat will—to curing urethane coatings before full cure is ger train coaches, where ulthe human eye—lose achieved, such as those applied to the exterior timate appearance as well much less gloss than a of storage tanks? as ease of cleaning, even high gloss coat with with strong cleaners, is reearly exposure to conquired. densation. Apparent Less well known coating damage to the semi-gloss is therethan those used with other coating applications for polyurethanes take materials such as epoxies. This fore less. Additionally, the underlyadvantage of a fast curing rate (for means that if a chemically cured ing epoxy coats still provide the example, coatings for splash and urethane is exposed to condensation bulk of the anti-corrosion protection. inter-tidal zones of marine struc- before it has cured sufficiently, the At the same time, modern chemitures) and tolerance of low tempera- curing agent will react irreversibly cally-cured urethane resins have ture application conditions. been formulated to give earlier resiswith the water instead of the base. In industrial maintenance, cost has The result will be a finish with a retance to water exposure—and, debeen a major limitation to the use of duced gloss level. This has occurred pending, on the temperature—rain thin film urethanes since their intro- on a number of structures in the showers after 4 hours have no meaduction. Even today, on storage open. Typically, the high gloss uresurable effect on the gloss at 68 F tanks, cheaper alternatives such as thane topcoat has been applied in (20 C). water-borne or solvent-borne the late afternoon in temperatures The applicator must still pay attenacrylics are often used. Chemically around 50 to 59 F (10 to 15 C) or tion to the sensitivity to water of curing urethanes had their first in- lower, and during the evening, conchemically curing urethanes, includdustrial use as topcoats in systems densation has occurred (dew). The ing following procedures for proper based on epoxies. The epoxy was next morning, the finish is flat and storage, mixing, and application of the part of the system responsible has to be sanded before a further the coating material; assuring that for corrosion protection, and the coat of urethane can be applied to application and drying, occur during chemically curing urethane was used restore the desired appearance. Unproper climate conditions; and to add gloss and color retention that expected rainfall can lead to the avoiding condensation and other epoxies lack. For this reason, the same situation. means of water contamination in the chemically curing urethane has been total process. Nowadays, it is common for large applied mainly as a thin-film high structures such as storage tanks not continued Q ✍ Copyright ©1996, Technology Publishing Company FEBRUARY 1996 / 17 PROBLEM SOLVING FORUM Answer From Melvin Y. Zucker, Yale Corporation, Portland, OR: Since the 1980s, aliphatic urethanes have become the topcoat of choice for the exterior of storage tanks because of their excellent color retention, hardness, and ultraviolet resistance and, perhaps, most important, because graffiti can be removed from them easily with negligible damage to the coating. The most popular urethanes for the exterior of storage tanks are two-part aliphatic polyurethanes with either hydroxy-terminated acrylic or hydroxy-terminated polyester polyols. As every applicator has learned, the urethane coatings can be tricky to apply over large surfaces outdoors. When exposed to moisture before cross-linking, a urethane coating will 18 / Journal of Protective Coatings & Linings Fig. 1 - A change in temperature during cure of an aliphatic urethane application resulted in flattening. Photo courtesy of Melvin Y. Zucker flatten (lose its gloss) or turn milky, depending on the amount of moisture to which it is exposed. In an ideal situation, each mole of polyol hydroxyl (polyester or acrylic resin) reacts with each mole of isocyanate in the hardener. When water is introduced, the balance is disrupted, and unreacted polyol hydroxyls remain. Some manufacturers add isocyanate to compensate for a certain amount of moisture in a limited range. Resin to hardener ratios of 1 to 1 are less susceptible to moisture than ratios of 4 to 1. However, even ratios of 1 to 1 will tolerate only a limited additional amount of moisture. Fig. 1 shows an aliphatic polyester polyurethane, 1 part resin to 1 part hardener, that was applied to an elevated water storage tank that contained about 3 ft (1 m) of water in the riser. When the urethane was sprayed, conditions were such that the applicator did not notice any condensation on the epoxy-coated steel. As temperature conditions changed after application but before polymerization was complete, more condensation occurred, and, as shown, the urethane turned milky, and lost its gloss. When a tank is completely filled with water while being coated, the continued Copyright ©1996, Technology Publishing Company PROBLEM SOLVING FORUM effect may not be quite as noticeable, but it occurs to some degree. I like my projects to have maximum gloss and luster, so I never permit painting with content in the tank. Owners are willing to spend more for a system of epoxy primer and aliphatic urethane topcoat than they would spend for an alkyd system to enjoy the superior long-term attrac- tiveness that the system offers. Hence, I caution all applicators in the bid documents that flat spots, lap marks, overspray, or discoloration will not be acceptable, and they should take precautions to apply the topcoat in appropriate conditions. This means that they must take into consideration conditions that may not be addressed in manufacturer’s data sheets, such as wind, temperature, and humidity, even after the work is finished for the day. I have seen many urethane coatings go flat even after application under ideal conditions during the day. They lost gloss because marine air conditions changed in the evening before cross-linking was sufficiently completed. Some aliphatic urethanes are easier to use than others. It behooves contractors to find products within a generic class that are reliable in a wide range of weather conditions. ✍ Answer From Mike Mitchell, International Paints, Felling, UK: Early water resistance is an issue with almost all coatings. Before molecular weight build-up has occurred, there is water sensitivity, which may result in discoloration and other deficiencies. Water sensitivity is obviously dependent on the rate of cure reaction and thus is more likely to occur in cool, northern climates. Other factors influencing water sensitivity are the hydrophobicity of material and solvent type. The traditional long oil alkyd finish is probably the least susceptible of all convertible coatings (those that will cross-link, as distinct from the low solids lacquer-dry thermoplastics, such as chlorinated rubbers or vinyls). This characteristic of the alkyd is because the oil content of the alkyd is water-repellent . There is a somewhat different situation in high humidity situations. The phenomenon of amine bloom was well recognized in the early days of epoxies, when low molecular weight aliphatic amines were widely used and “greasy” surface 20 / Journal of Protective Coatings & Linings Copyright ©1996, Technology Publishing Company PROBLEM SOLVING FORUM layers were common on curing in high humidity and low temperatures. The amine bloom and the greasy oily surfaces were the reaction of free amine with atmospheric moisture and carbon dioxide. In chemically curing polyurethane coatings, premature loss of gloss because of high humidity is not, in my experience, a widely occurring phenomenon, especially in Europe. Most comments on this in my company have emanated from the regions of South East Asia and areas such as Papua, New Guinea, to the north of Australia. Surprising, there have been few comments from the Gulf of Mexico, which, at first sight, would have similar climatic conditions. My belief is that in Europe much polyurethane used at new construction is applied inside fabricating shops in controlled conditions. With polyurethanes, the reason for the problem lies in the cross-linking chemistry and in the move to higher and higher solids, where lower molecular weight polyols, typically acrylic polyols, are used. With the higher molecular weight polyols used in the 1960s and 1970s, considerable lacquer drying occurred. With the greater number of hydroxyls on each chain, a further rapid increase in the glass transition temperature (Tg) occurred because of the increased molecular weight. The rapid increase in Tg improved both early water resistance and resistance to high humidity. With the onset of higher solids, overcoatable acrylic polyurethanes with a lower number of hydroxyls per chain, lower molecular weight resulted, and the above features were lost. The opportunity for alternative reactions to occur then became much greater. The principal (and the desired) reaction with two-pack acrylic urethanes is the following: an acrylic polyol and a diisocyanate combine Copyright ©1996, Technology Publishing Company to form a polyurethane. However, in all circumstances, there is a potential competing reaction, that between the diisocyanate and atmospheric moisture. In this reaction, a polyurea is formed, which in itself is not necessarily a major problem, but the byproduct of the reaction, carbon dioxide, is a major problem. When carbon dioxide forms at the surface, small defects occur that detract from the overall gloss. The reaction with polyol is preferred and obviously can be accelerated by use of traditional accelerators such as dibutyl tin laurate, but such additives severely shorten the pot life of the product. The most practical solution in areas with high evening humidities is to paint in the morning and allow curing to take place in the afternoon before the onset of high humidity. ✍ Reader Response On Inspecting and Repairing Concrete before Lining From A.W.M. van den Hondel, Nebest, Groot-Ammers, The Netherlands: The November 1995 JPCL posed the following Problem Solving Forum question: “What types of inspection and repair procedures are needed to ensure that a previously uncoated concrete structure is capable of accepting a lining?” Concrete is a relatively inexpensive building material with structural properties that make it capable of supporting loads. However, concrete can corrode when it is in contact with aggressive chemicals such as fertilizers, manure, acids, sulfates, and salt water. Protecting it with an impermeable lining is a straightforcontinued on page 137 FEBRUARY 1996 / 21 PROBLEM SOLVING FORUM continued from page 21 ward way of preventing deterioration. A thorough investigation of previously uncoated concrete is needed to determine whether it is capable of accepting a liner. In the inspection, one must assure that the following conditions are met. • The concrete has attained at least 80 percent of its design strength. • The concrete quality at the surface is such that it can withstand the occurring mechanical loads. (If not, a liner will not do any good.) • The structure must not be open to water; water penetration into a structure with an impermeable lining can cause problems. • The surface must be cleaned of alkali, efflorescence, sand (from construction), dust, dirt, oil, grease, fat, chemical films in general, curing compounds, form release agents (containing waxes, silicones, or silicates), laitance, loose deposits, rust, and (for most linings) water. These contaminants may be removed by abrasive blasting, mechanical scraping, or chipping. For example, for new floors, light sweep blasting is usually the most effective way to remove laitance. All dust must then be removed. Oily contamination may be removed using an emulsion cleaner, but heavily contaminated floors may require specialized techniques such as flame cleaning. • Spalling and delamination of concrete must be repaired. Precautions should be taken to assess the nature and origin of the deterioration process. Is it, for example, rebar corrosion (chloride vs. carbonation), chemical corrosion of the concrete, or an alkali aggregate reaction? • Cracks larger than 100-200 micrometers (4 to 8 mils) should be filled by resin injection. Whether or not the injected polyurethane (or epoxy) should be flexible will depend on the origin of the crack Copyright ©1996, Technology Publishing Company (e.g., structural or settlement causes, temperature cycle deformation, accidental overload, or hardening shrinkage). • Small holes or voids in the concrete surface should be filled with a suitable filler compound. Once the necessary cleaning and repair procedures have been performed, the concrete is ready for lin- ing in accordance with the manufacturer’s instructions. On the Advantages and Disadvantages of Partnering From Andrew Greig, Kvaerner Installasjon a.s., Norway: The December 1995 JPCL raised the continued FEBRUARY 1996 / 137