Influence of Layered Double Hydroxides on Polyaddition of Aromatic Isocyanate and Macrodiol

Layered double hydroxides (LDH), hydrotalcites in particular, represent a major and versatile class of 2D materials, which can be used to encapsulate drugs, flavors and other functional components in various matrices [1]. Consequently, LDH finds innovative uses in diverse polymer applications. However, its usage in polyurethane (PUR) might be complicated, because LDH contains hydrated moieties, which might react with isocyanates and disrupt polymerization. In this study, hydrotalcite LDH as MgX[Al2(OH)2](2+X)CO3 (x=4-6) with relatively low H2O contents (<0.5% wt. loss in 2 hrs at 105°C) was incorporated into PUR by mixing it in a diluted OH-terminated prepolymer and curing with aromatic isocyanate. The prepolymer was produced in ethyl acetate by chain extending the ether-ester macrodiol of 2700 g/mol, synthesized from adipic acid and diethylene glycol, with hexamethylene diisocyanate at 1:0.3 mol. ratio [2]. Polymerization was carried out by mixing the prepolymer with a trifunctional aromatic isocyanate (prediluted with 25% wt. ethyl acetate with or without LDH) at 1.4 mol. excess. Three mixtures were studied at 50°C:
• no LDH (60% wt. prepolymer, 32.5% wt. ethyl acetate and 7.5% wt. isocyanate); • 1.8% LDH (59% wt. prepolymer, 31.7% wt. ethyl acetate, 1.8% wt. LDH and 7.5% wt. isocyanate) and • 6% LDH (56% wt. prepolymer, 31.5% wt. ethyl acetate, 6% wt. LDH and 7.5% wt. isocyanate). In every mixture, the isocyanate was preheated and added as the last component immediately before testing. The mixtures were agitated for 0.5-1 min and poured into a capillary Cannon-Fenske viscometer of 500 size (orifice of 2 mm ID), equilibrated at 50°C. The rate of isocyanate polyaddition to the OH-terminated prepolymer was monitored by measuring kinematic viscosity using an adapted ASTM D445 procedure.

The results show, Fig. 1, that viscosity builds up gradually due to the ongoing polyaddition. Inclusion of 1.8% wt. LDH increases the viscosity by nearly 50%, retaining this ratio throughout the course of polyaddition. Higher amounts of LDH increase viscosity further, as in case of 6% wt. LDH. Nevertheless, the rate of thickening remains similar, which implies that LDH addition does not change polymerization mechanism significantly. However, H2O concentration in LDH should be carefully considered, because the tested hydrotalcite had relatively low moisture levels.