Principles of Urethane Modification

Principles of Urethane Modification. The flame retardance and temperature resistance (or flame endurance) of modified isocyanurate foams are affected by the following factors  

In contrast, TDl-based modified isocyanurate foams did not show any increase in bum-through time, even if the NCO/OH equivalent ratio was increased. This significant difference could be attributed to the difference in flash point of the two isocyanates. The flash points of polymeric isocyanate and liquid-modified MDI oligomers are 200°C, and that of TDl is 135 C. 

When urethane linkages dissociate by dry heat or flame the dissociation of the urethane linkage starts at temperatures below 200°C, which temperature is already higher than the flash point of TDI. Therefore, in the presence of air, the dissociated TDI vapor ignites immediately. 

In contrast, polymeric isocyanate does not immediately ignite, because its decomposition temperature is lower than its flash point. Accordingly, only polymeric isocyanates or liquid oligomeric MDI should be used for urethane-modified isocyanurate foams having substantial flame retardance. 

The second factor important in obtaining highly flame-retardant isocyanurate foams is the NCO/OH equivalent ratio. Fire endurance, i.e., flame retardance) and temperature resistance, can be increased with increase of the NCO/OH equivalent ratio when polymeric isocyanate is used as the polyisocyanate component. 

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