Polyurethane systems are present in our lives in dozens of forms. However, there are still those who question the fire behavior of this insulating material. Among the isocyanate-based foams, polyurethane foams, both flexible and rigid, are inherently flammable. Proper use of flame retardants can result to some extent in combustion-modified urethane foams. The mechanism of combustion is schematically shown below:
The principle of flame retardation is to stop the combustion cycle as shown above. In view of the combustion mechanism, the principle of flame retardation can be drawn as follows:
1. By adding char-forming agents, that is, dehydration agents such as phosphates that can reduce generation of combustible gas
2. By adding radial scavengers, such as, Cl- or Br-compounds, because the combustion reaction is a radical reaction.
3. By adding fillers such as alumina trihydrate to reduce calorific value of combustion
4. By incorporating more thermally stable linkages such as isocyanurate linkages to reduce thermal decomposition (polyisocyanurate foams belong to this category)
What is the toxicity of polyurethane fumes?
Polyurethane is a material of organic origin and therefore combustible. If it is directly affected by a fire, the fumes generated during combustion have a composition similar to that of other organic products used on a daily basis, such as wood, cork or cotton. In addition, in order to avoid fire damage to the building structures, polyurethane systems are protected by other materials that are more resistant to fire, such as concrete, brick, plaster, mortar, etc.
If the fire reached such proportions that this protection would give way, the polyurethane systems, when dealing with a material of organic origin, burn, but with a particularity: polyurethane does not melt or drip like other plastics (for example polystyrene), but the surface coming into contact with the flame carbonizes and protects the core, thereby maintaining some structural stability for a certain period of time. In many cases, it is heard that the origin of fires are plastic materials, like polyurethane, that are used in the isolation of the building, but this is certainly not true. Polyurethane has the peculiarity that when it comes into contact with the flame, instead of melting, it carbonizes, protecting the core of the fire. This causes the structure to remain stable for some time. For this reason, polyurethane systems are never the origin of a fire. The beginning must be a different one and the isolation, if achieved, will be based on the design of the structural element in which it is integrated, and the time that elapses as the fire develops. The design of the building is key when it comes to fire safety.
Polyisocyanurate foams of specific types are highly flame-retardant and heatresistant and can meet the increasing requirements of building codes. However, unmodified polyisocyanurate foams are extraordinary friable and therefore cannot be used for practical applications. Polyisocyanurate foams modified by urethane-linkages. The foam is remarkably flame-retardant and stable at high temperatures and is low in friability, so the foam has been used for highly flame retardant applications such as petrochemical plant insulation. This is particularly true of polyurethane foam produced from polyester polyols. These polyols tend to degrade under humid conditions and to degrade quickly when thermal decomposition or hydrolysis products of flame retardants are present. It is also true that polyurethane foams for certain applications need to be resistant to smoldering ignition due to contact with a cigarette or other ignition source. In addition, in the case of polyester-based polyurethane foams, hydrolytic stability of additives is another important criterion that must be met.
The wide range of insulation products manufactured with polyurethane systems not only complies with current energy efficiency regulations, but they also meet European fire resistance standards. The polyurethane products reach between B1 to B3 in the Euroclass classification.
Polyurethane foams are used primarily for insulation and cushioning. Since they are thermoset polymers, additives such as flame retardants are generally incorporated during polymerization. Equipment for producing polyurethane foams is generally designed to handle liquids, discouraging the use of solid flame-retardant additives. Polyurethane foam is normally flame retarded by incorporating phosphorous and/or halogen-containing compounds into the formulation. Since these flame retardants are nonreactive additives, they are not permanently bound into the polymer and, hence, have the tendency to migrate to the surface of the foam. This results in a loss of flame retardance. Flame retardants are classified by different types, including, liquid, powder, reactive, nonreactive, radical scavenger, char-forming, noncalorific additives, and incorporation of more thermally stable linkages such as the isocyanurate linkage.