Sprayed polyurethane foam (SPF) systems are manufactured from a chemically complex mixture of polymeric materials such as liquid isocyanates and liquid polyol resins, which are added with various cell stabilizers, blowing agents, combustion-retarding agents, catalysts and fillers. The polyurethane foam is developed by the heat reaction between the liquid isocyanates and liquid polyol resins, coupled with the vaporization of the blowing agent. This reaction forms cells that change the material from a liquid to a cellular mass. The material is spray applied over the existing substrate.
SPF systems provide many advantages on low-slope roof systems. The primary advantage is that the material has a high thermal resistance value that increases per unit thickness. The material is lightweight (less than 1 psf for a 3-inch thickness), a distinct advantage for remedial recover applications. The application method is seamless, and with proper maintenance the system can provide a long-term life cycle in the proper environments. The long-term success of the system is dependent on the SPF coating.
SPF systems require coating applications for long-term service performance. The primary reasons for coating applications are (1) SPF is not ultraviolet resistant, (2) SPF is not waterproof, and (3) the coating helps the system resist damage from heavy traffic and hail. Typical SPF elastomeric coatings are urethane, acrylics or silicones.
The coating must possess the physical property characteristics that the SPF is deficient in. These properties include:
- Tensile strength.
- Tear strength.
- Impact resistance.
- Abrasion resistance.
- Chemical resistance.
Coping With Thermal MovementTensile strength and elongation are required to provide the SPF with the required resistance to thermal movement, which can create splits and openings in the SPF surface. Elongation capacity should be over 100 percent as tested under ASTM D412. Tear strength and impact resistance are also important characteristics. SPF - on its own - does not posses the strength to resist heavy foot traffic, damage from dropped tools or hail. Minor punctures or tears resulting from these conditions can lead to moisture infiltration in the system. Impact resistance can be determined by conducting ASTM D2794 impact resistance tests.
The coating must also provide high reflectivity. SPF is not ultraviolet resistant and is known to oxidize rapidly in sunlight. The degree of degradation depends on the degree of exposure. Overexposure is illustrated by changes in the SPF color. The SPF goes from its original cream color to a tan color under moderate exposure conditions and elevates to an orange color when it is in its most extreme, friable stage. If the condition is not treated, the erosion can continue through the total SPF cross-section.
Polyurethane foam is vulnerable to ultraviolet attack and weather degradation. A significant number of SPF failures occur due to the loss or erosion of the polyurethane foam’s protective coating. When the protective coating bond is broken, deterioration can occur from moisture infiltration at openings or splits from expansion and contraction.
Even if roof leaks are not occurring, the loss of insulation worsens over time. This condition can occur from aging and natural weathering of the material or excessive water flow. Abrasion can also occur from excessive or from birds pecking at the foam surface to feed on the insects that migrate into the system. The loss of coating is revealed by a brownish-yellow color of the polyurethane foam.
Another point of concern with uncoated SPF is that it is not an inherent waterproofing material because of its closed cell structure. If the material is not coated with a waterproofing coating, SPF is vulnerable to moisture infiltration in areas of ponded water or in high-humidity environments. Moisture infiltration into the system decreases the thermal and structural integrity of the SPF.
Coating ApplicationCoating applied on SPF systems is typically a liquid elastomeric coating that is applied in two coats. The base coat is applied on the same day as the SPF application. The base coat is typically darker than the final coat so that the applicator can readily detect the areas that have already been coated during the final coating process. The coating should be applied in a uniform and even application with the thickness determined by the texture of the foam surface. More coating is required for irregular surfaces than for even surfaces. After the coating is applied and cured, the applicator should inspect the area for thin coating applications, pinholes, blisters and cracks. Proper repairs should be completed prior to departure from the project.
On remedial projects, coating application is most acceptable on smooth surfaces. Surfaces that illustrate orange peel or coarse orange peel characteristics are also acceptable, but may require more material for adequate coverage. Surfaces that illustrate a rippling condition are marginally acceptable; however, they require a minimum of 5 percent more coating to fill in all irregularities and voids in the surface. Surfaces that illustrate popcorn or tree bark conditions are unacceptable for coating applications. The irregularities of these surfaces are prone to pinhole openings and voids. A minimum of 50 percent additional coating is typically required after repairs to all openings are completed. The economics of these procedures may warrant replacement.