Biurets temperature effect

Details of the TDI and MDI foam model systems have been previously published [2]. The models require the use of mono-functional reactants that are quantitatively analysed to correlate structure-activity relationships for various classes of catalysts. A realistic thermal profile is produced through the imposition of an external exotherm. Urethane, urea, allophanate and biuret reaction products are quantified by liquid chromatographic analysis of quenched reaction samples. The models effectively account for such nonideal conditions as reactant depletion at variable rates, temperature and concentration-dependent catalyst activity, and catalyst selectivity as a function of isocyanate distribution. 

Temperatures of reaction can, of course, be important. At up to SC C the linear chain-forming reaction predominates but as higher temperatures (up to 150°C) are reached then biuret and isocyanurate formation become effective and branching occurs. At above 150°C some of the less stable links are affected and reversion or degradation can then take place. It must be stressed that the isocyanate reactions are highly exothermic, and under conditions where heat transfer is slow appreciable temperature rises can be experienced care is necessary to minimize their occurrence, otherwise deterioration in properties results. 

Increased temperature also has an important effect on the properties of the cured polymer since both the allophanate and biuret linkages tend to dissociate at higher temperatures giving a more linear polymer. Some results are given in Table 4.5. 

---