Flammability rigid foams

Commercially, polymeric MDI is trimerized during the manufacture of rigid foam to provide improved thermal stability and flammability performance. Numerous catalysts are known to promote the reaction. Tertiary amines and alkah salts of carboxybc acids are among the most effective. The common step in all catalyzed trimerizations is the activation of the C=N double bond of the isocyanate group. The example (18) highlights the alkoxide assisted formation of the cyclic dimer and the importance of the subsequent intermediates. Similar oligomerization steps have been described previously for other catalysts (61). 

A number of flame retardants for rigid urethane foams have been developed over the past 30 years. Nevertheless, substantially flame-retardant, and fire-resistant rigid foams are not available, because the urethane linkage is thermally unstable and decomposes to produce low-molecular-weight flammable compounds. 

Some polymers are easier to foam than others. Indeed, it was not until methods were found to circumvent the inclusion of cells in the early history of the phenol formaldehyde polymer that it gained any commercial significance. The development of foamed phenolic resins only became important much later when a specific need arose to produce rigid foam with reduced flammability. This consideration also led to the development of polyisocya-nurate foams and carbodiimide foams. On the other hand, the polypropylene family of polymers, although having a tonnage far exceeding that of phenol formaldehyde resins, is... 

Table 7.6 Flammability data of rigid foams with 35 kg/m density 153]...

Flammability. Rigid polyurethane foams, along with other organic plastics, have been the subject of much work in the field of flame retardance. A variety of phosphorus and/or halogen containing additives (reactive and non-reactive) are now available which improve the flame resistance of polyurethane foams. The choice of additives and of test methods is however best decided upon in the light of the application envisaged. Table 7 shows some typical properties. 

Bonsignore, P.V. Levendusky, T.L. Alumina trihydrate as a flame retardant and smoke suppressive filler in rigid high density polyurethane foams. J. Fire Flammability 1977, 8, 95-114. 

Usually, different methods are recommended for each particular type of material (film, fabric, carpet, rigid plastic foam, elastoplastics, etc.) to characterize its flammability adequately. Test procedures are divided into small-, medium-, large- and real-scale experiments, depending on the sample size. 

Kopshev, E. Yu, Shoshtaeva, M. V., Korolkov, L. L Effect of apparent density on flammability of rigid polyurethane foams. Plast. Massy 1979, No. 2, 51 (in Russian)... 

Flame Retardants. Among the isocyanate-based foams, polyurethane foams, both flexible and rigid, are flammable. Due to serious fire hazards of polyurethane foams, strict fire regulations have come out on the use of foams in the areas of furniture and public transportation. In addition, the use of rigid urethane foams in building insulation have resulted in stricter fire regulations. 

The high flammability and toxic-gas generation of flexible and rigid urethane foams have been major problems in the urethane-foam industry, and accordingly considerable efforts have been focused on the production of substantially flame-retardant flexible foams. 

Polyvinyl Chloride (PVQ Foams. Rigid PVC foam is inherently fire retardant because of the high chlorine content (56.7%). Flexible PVC foams present increased fire hazards because of the plasticizers they contain. Flammable plasticizers used include alkyl phthalates, as dioctyl phthalate. Non-burning types include alkyl aryl phosphates (phosphate esters). The latter types should improve resistance to ignition and reduce flame spread when compared to the usual phthalate plasticizers (38) (41). 

A number of methods have been devised for producing cellular products from PVC, either by a mechanical blowing process or by one of several chemical blowing techniques. PVC foams are produced in rigid or flexible forms. The greatest interest in rigid PVC foam is in applications where low-flammability requirements prevail. It has an almost completely closed cell structure and therefore low water absorption. The rigid PVC foam is used as the cellular layer of some sandwich and multi-layer panels. 

Polyurethanes occur as rigid or flexible foams, elastomers, thermoplastics, lacquers, adhesives and can be manufactured as synthetic leathers. The rigid and flexible foam materials applied in bulk give rise to the biggest flammability problems. 

A large number of patents are concerned with the flame-retardance of polyurethane foams. Techniques for reducing the flammability characteristics of rigid polyurethane foams were commercialized quite early on most of their grades have been flame-retardant since the early sixties. However, effective flame-retardance of flexible polyurethane foams is still a problem, as the retarding agents may reduce the durability of the products. 

Hilado conducted a comparative study on the flammability of great many flame-retarded polyurethane foam grades. The flammability of rigid polyurethane foams are markedly reduced by formation of an isocyanurate structure. The exploitation of this possibility is detailed in Section 5.1.5. 

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