Feature All images courtesy Garland Co. Specifying Polyurethane Products Understanding the chemical bonds that bind By Jason Smith F or many people, the promise of polyurethanes was proven in the paints protecting patio decks from the rain.1 These reactive waterproof systems have long since found their way from the backyard to the roof in many different forms—foam insulation, coatings, and various sealant and adhesive products. Despite the material’s seeming ubiquity, not all design/construction professionals understand the building science behind these products. Knowing polyurethane attributes helps ensure products are properly specified. Polyurethane foams Introduced to the roofing realm as a direct response to the oil embargo, polyurethane insulation was intended as a way to curb heat loss and rising utility costs.2 The polymers are very strong when manufactured into a closed-cell rigid foam. As the material is heated to a much higher temperature than usual during manufacturing, the urethane groups react with themselves to form a very strong, crosslinked network called an isocyanurate. The polyurethane foams have a very high R-value, which led to polyisocyanurate boards becoming the most popular insulation, regardless of whether it is a single-ply or built-up roof (BUR). It was discovered when a 51-mm (2-in.) layer of polyurethane foam was applied to a roof deck, water movement was restricted to the hole—even if the hole was punched through the entire 51 mm. 40 January 2010 Beginning in the late 1960s and early 1970s, polyurethanes were sprayed onto roofs in the form of two-component foams called sprayed polyurethane foams (SPF). When mixed in a specified ratio—typically 1:1 by weight—a chemical reaction occurs, expanding the SPF up to 30 times its size and forming a solid, seamless, closedcell, fully adhered roof system for residential, commercial, institutional, or industrial projects.3 With polyurethane foams, seams around flashings and other protrusions are sealed, eliminating a common source of leaks. SPF roofs provide excellent water resistance and thermal insulation capabilities. The foam coating itself is lightweight, typically 22.7 kg (50 lb) per square (i.e. 9.3 m2 [100 sf]), versus 363 kg (800 lb) per square for a BUR or about 500 kg (1100 lb) per square for a ballasted single-ply roof.4 However, despite excellent insulating properties, sprayed polyurethane foams have their limitations. The need for keeping reactive raw materials separate means special equipment must be used to bring the components together in the required ratio. If improperly mixed, the product either does not cure (too much polyol) or does so too slowly with little or no foaming (too much diisocyanate). Due to respiratory risks associated with scattering foam during application, this author heavily advises operators wear respirators fitted with special diisocyanate filters to optimize worker health and safety.5 Construction Canada CC_JanFeb10.indd 40 12/22/09 11:56:03 AM