Influence of Aliphatic Moieties in Diisocyanates on Chain Extension Kinetics of Adipate Macrodiols

Plastics, adhesives, resins and other polyurethane (PUR) formulations are widespread in various industrial, food, biomed and other applications [1]. In many cases two component blends are used, where “OH-component” represents a viscous prepolymer, which can be crosslinked with isocyanates into fully cured PUR. Adipate macrodiols are often employed in manufacture of non-yellowing and other high performance PUR. Their chain extension must be accurately controlled in order to obtain a needed molar mass distribution [2]. In this study, an adduct of adipic acid with ethylene glycol was used as an ester type macrodiol, while an adduct of adipic a. with diethylene glycol as an ether-ester type macrodiol, Fig. 1.

Fig. 1. Structures of adipate microdiols (n ~ 12-14), used for chain extension with diisocyanates

Chain extension of excess macrodiols was performed at 2:1 and 3.33:1 molar ratios with respect to aliphatic diisocyanates: either hexamethylene diisocyanate (HDI) or isophorone diisocyanate (IPDI). Titration of polymerization mixture aliquots with dibutyl amine was exercised to monitor the amount of unreacted NCO (isocyanate) groups, Fig. 2.

Fig. 2. Isocyanate (NCO- group) depletion at 80°C during chain extension of adipate macrodiols in 2:1 or 3.33:1 molar excess with aliphatic diisocyanates HDI (left) and IPDI (right).

Isocyanate depletion appears to follow approximate semilog dependence on the reaction duration, as might be inferred from satisfactory least square fits. R² values exceed 0.95 for all chain extensions, except for IPDI at 1:3.33 ratio to the ester macrodiol. Overall, IPDI seems to react slower than HDI, most likely due to steric hindrance, although the presence of one secondary alpha α-C atom might also affect the reactivity of NCO group in IPDI. Established semiquantitative trends might be beneficial for better control of prepolymer production in industrial manufacturing.

Acknowledgment. This study was carried out under project TERMINUS, funded by the European Union under Horizon 2020. Call: H2020-NMBP-ST-IND-2018. Grant Agreement: 814400.