Today's roofing contractors and building superintendents are facing a growing and sometimes bewildering array of options for sealing and protecting metal roof decks. Over the past few decades, traditional technologies such as asphalt and single-ply, rubber-based sheet goods have been augmented by a plethora of seamless, monolithic membranes.

Choosing the right materials to work with, all of which have their particular strengths and weaknesses, can have a dramatic impact on the whole life cost as well as the long-term success of the project.

Recent developments in polyurethane hybrid thick-film coatings have created an exciting new option for protecting metal roof decks that allow the contractor or superintendent to solve many of the problems and key failure issues associated with these alternative materials.

Table 1. Click image for larger view.

Background

Acrylic emulsion elastomers are widely used as seamless roof linings because of the excellent weatherability of the cured material, UV stability and ease of application. The key failure issue associated with acrylic membranes is poor resistance to ponding water, which often results in blistering and delamination of the coating to the roof deck in those areas. With tensile strength in the 300 psi range and elongation in the 120% range, cracking and damage to the membrane can occur due to heavy traffic or the continuous cycle of expansion and contraction of the roof deck.

Although the cured acrylics exhibit excellent weatherability, the water-based emulsions are very vulnerable to rain, moisture and rapidly changing temperatures during the curing process. The finished lining usually consists of two separate 15 mil layers with a polyester reinforcing fabric between them. Most manufacturers recommend at least 12 continuous hours of ambient temperatures greater than 50 °F after application and a minimum of 24 hours to properly cure each layer. This makes the applicator much more dependent on prevailing weather conditions.

Liquid EPDM membranes are also used to provide a UV-stable, water-resistant lining that maintains its properties over a wide temperature range. One key issue with this material is a 24-hour cure time, a 50-hour walk-on time and a full cure within three to seven days, which can lead to significant down time. The ambient temperature must also be greater than 55 °F or the curing process will stop indefinitely until the temperature rises above that level. Another drawback is that the material is relatively expensive. At $52 per gallon and, with 2.4 gallons needed to produce the required 20-mil dry film thickness per square, the material cost can exceed $125 per 100 foot square.

Silicones are most often used to provide a waterproof membrane over insulating polyurethane foams. They are highly flame retardant and can maintain their properties at temperatures ranging from -70 to 320 °F. One drawback is that they are solventborne and must be applied in two or more coats to ensure pinhole-free continuous film, and there is a 3- to 4-hour dry time for each layer at an ambient temperature of 75 °F. In cooler weather it can be significantly longer. They also exhibit poor physical properties, which can result in damage to the membrane from normal wear and tear.

Polyurea spray systems have become increasingly popular over the last decade, most notably because of their rapid cure, tough properties and the fact that they contain no VOCs. The extremely high reactivity of these systems, which is the principle reason behind their insensitivity to humidity, is not without some drawbacks. A 1- to 5-second gel time will greatly limit surface wet out, which can result in adhesion issues. These systems can also be difficult to process because the material is literally reacting as it is leaving the spray gun. Table 1 shows the comparison of various systems.

Table 2. Click image for larger view.

Chemistry is the Key

The difference between a polyurea and a polyurethane hybrid can be traced to the type of isocyanate reactions that take place. A polyurea results from a reaction between an isocyanate-terminated prepolymer and a resin blend containing only amine-terminated resins and chain extenders. The blazing speed of this reaction, which is far more rapid than the isocyanate/water reaction, is the primary reason polyureas can be sprayed even in cold or highly humid conditions without foaming.

The resulting urea linkage is sometimes referred to as a hard-block. These hard-block units have a high affinity for each other and tend to agglomerate into a highly ordered structure where they develop hydrogen bonding between them. This is what leads to the high stiffness, tensile and tear strength and chemical resistance of polyureas.

A polyurethane hybrid thick-film coating is a polymer composed of both urea and polyurethane linkages, which results from the reaction of the isocyanate with a resin blend containing a mixture of amine-terminated and hydroxyl-terminated resins.

The hydroxyl/isocyanate reaction produces a urethane linkage or soft-block. These soft-block segments tend to phase separate from the urea hard-block segments during the reaction, which produces a polymer that exhibits the best of both worlds: the excellent properties of polyureas combined with improved elasticity, low-temperature flexibility and greater surface wet-out and adhesion to the substrate due to a slightly slower reaction rate.

Table 2 shows the comparison of polyurethane hybrid thick-film coatings to polyurea spray systems.

Equipment Considerations

The ability of the machine to heat each component independently to a minimum of 160 °F prior to mixing in the gun is essential. These are the temperatures necessary to reduce most prepolymers and resin blends from a room temperature viscosity greater than 1000 cps to a working viscosity of 100 cps. This is important not only for the machine to supply an adequate flow of material to the gun through several hundred feet of hose but also for the gun to provide an optimal mix. Independent control allows the operator to fine tune the viscosities and pressures to ensure the best mix.

High-pressure impingement mixing guns fall within two general categories. Mechanical purge guns have a fixed mixing chamber. When the gun is triggered, a valving rod pulls back, allowing both components to impinge directly into each other at high pressure. The mix is then completed as the material squeezes through the restricted orifice of the spray tip. When the trigger is released, the valving rod moves back and clears out the mixing chamber. Air purge guns have a moving mixing module/valving rod combination. Because of this, purge air is necessary to clean out the mixing chamber after the trigger is released.

One of the most important factors to keep in mind when selecting equipment is the gun output to machine output ratio or GO/MO. To avoid mixing problems, this ratio should be less than 0.75 to ensure that the machine can supply an adequate flow rate for the mixing module and spray tip to function properly.1 For example, with a machine capable of 12 lbs per minute at 2000 psi you would want to select a mixing module and spray tip combination from the gun manufacturer that will produce an output of less than 8 lbs per minute at 2000 psi to ensure an optimal mix.

Another important parameter is keeping both fluid pressures greater than 1800 psi at the gun and balanced within 10% of each other during spraying. With independent temperature controls for the resin and the prepolymer sides this is fairly easy. Greater pressure differentials can lead to mixing issues which, depending on the severity, can produce symptoms ranging from poor properties to blistering to partially cured material.

If an air purge gun is selected, it is critical to ensure that the air supply to the gun is dry and free of oil contamination. The air supply must be conditioned with a desiccant and an oil trap to prevent water and oils from being introduced into the spray pattern during triggering. Furthermore, the air supply must be at an adequate volume and pressure as specified by the manufacturer for the gun to work properly.

Surface Preparation

The first step in the process is to carefully inspect the roof deck to make sure it is structurally sound. Any deteriorated, damaged or corroded areas must be repaired. Significant gaps between seams, joints or protrusions or loose fasteners should be repaired as well. Any cooling systems capable of producing condensation on the roof deck must be disconnected.

The second step is to thoroughly clean the roof deck with a high-pressure wash containing an alkaline detergent in accordance with SSPC-SP1 and SSPC-SP2 to remove any oil, dirt, grease, loose mill scale or rust that can interfere with adhesion of the thick-film coating to the surface. After treatment, the deck must be rinsed with fresh water and allowed to dry.

The third step in the process is to apply an approved epoxy primer to the deck surface in accordance with the manufacturer's instructions. It should be noted that the installation of other monolithic membranes requires similar preparation and priming. As soon as possible after the primer has dried, application of the polyurethane thick-film coating should commence. The most important consideration at this point is that the surface temperature of the roof deck be at least 5 °F greater than the dew point.

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Recent Case Studies

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Conclusion