Fundamental differences in the insulation performance of rigid foams can stem from the crosslinked network or differences in crosslink density of the polymer structure, and the related gelation and vitrification processes. Higher functionality isocyanates can trigger gelation and vitrification processes earlier in the foaming process relative to a lower functionality isocyanate, which can result in smaller size cells, and ultimately better insulation performance. For example, polyisocyanurate foam (PIR) from higher functionality polymeric methylene diphenyl diisocyanate (MDI) such as Dow Papi 20 polymeric MDI can show at least 1.0-1.5 mW/(m-K) lower k-value than foam synthesized using lower functionality pMDI such as Dow Papi 27 polymeric MDI. In this paper, we discuss the chemical modification of Papi 21 polymeric MDI by synthesizing isocyanate-capped prepolymers that show improvement in k-value without sacrificing mechanical properties. Foaming systems from prepolymers show some unusual characteristics that can be attributed to modifying the gelation and vitrification processes in the foaming reaction. These modifications can control cell growth and ultimately lead to smaller cells, which is generally a desirable attribute for lower k-value.
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