- Spray polyurethane foam is produced by combining two components, called Side A and Side B, that are mixed in about equal volumes. Both contain monomers, which are chemicals that can bond together to make long-chain molecules called polymers. Side A is composed of isocyanates, a highly reactive group of chemicals that are characterized by the isocyanate group, N=C=O, or a nitrogen atom double-bonded to a carbon atom double-bonded to an oxygen atom. Isocyanates react with polyols contained in Side B to form the polymer polyurethane. Polyols are chemicals that contain multiple hydroxyl groups, or an oxygen atom attached by a single bond to a hydrogen atom. For insulating foam, polyols are derived mostly from petroleum sources, with some manufacturers using castor oil or soybean oil as polyol sources. Side B also contains other ingredients. Foam manufacturers have their own proprietary blends of ingredients for Sides A and B.
- Many types of isocyanates are used in the manufacturing of spray polyurethane foams. The most widely used are methylene diphenyl diisocyanate or MDI, toluene diisocyanate or TDI, hexamethylene diisocyanate or HDI, and MDI-, TDI- and HDI-based isocyanates. Diisocyanate chemicals contain two isocyanate groups. Isocyanate compounds are used in many products besides foam, including automobile body repair, paints and varnishes, fibers and elastomers.
- Besides polyols, Side B also contains chain extenders, which are special polyols containing short chains of atoms that help connect the longer chains of the forming polymer. Catalysts are present that facilitate the reaction between the isocyanate and the polyol. Surfactants control the foaming that takes place when the monomers combine so that the foam has the desired structure. Finally, there is a blowing agent that actually causes the foam. Different blowing agents are used by manufacturers, and include water, carbon dioxide, pentane, or other chemicals. Without a blowing agent, the polyurethane would turn out to be a solid substance, not a foam. Flame retardants might also be included in the mix.
- Once spray polyurethane foam cures and hardens, most of the chemicals it contains do not continue to volatilize or outgas. However, workers who apply the foam are exposed to isocyanates, which can irritate the body's mucous membranes in the eyes, digestive system, and respiratory tract. Exposed skin can become severely inflamed. People can become sensitized to isocyanates and develop severe, even life-threatening, asthma, according to the Centers for Disease Control and Prevention. For Side B chemicals, catalysts may contain amines which can cause blurry vision. Some flame retardants can contain toxic chemicals, some surfactants may have connections to endocrine disruption, and some blowing agents can be harmful to health. State and national agencies are working to produce guidelines that will prevent health problems. Everyone who applies a spray polyurethane foam, from the contractor to the do-it-yourselfer, should become familiar with safety procedure.
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