MODELING POLYCONDENSATION EQUILIBRIUM FOR NYLON 6 AND NYLON 6,6

摘要

A new model equation is developed to describe the complex influence of water W and temperature T on the condensation equilibrium of commercially important nylon polymers (i.e., nylon 6, nylon 6,6 and nylon 6/6,6 copolymer). As shown in Figure 1, the apparent nylon 6 equilibrium constant K_6 is small at very low [W] (e.g.,[W] < 0.1 mol kg~(-1)), increases to a maximum value for [W] near 1 mol kg~(-1) and then decreases with increasing [W]. Also, K_a decreases with increasing T. This complex behaviour of K_a has long been a topic of mystery and practical concern because reliable values of K_a are important for simulating and optimizing nylon reactor systems. Until now, no literature models have been able to capture the overall trend of nylon 6 and 6,6 equilibrium data. A new model equation is developed based on the extended nylon 6 reaction scheme in Table 1, where the polycondensation reaction R1 involving amine ends A, carboxyl ends C, amide links L and W is accompanied by four additional reactions involving amidine ends I and hydrated carboxyl ends C*. Amidine ends form via the reverse of R2 and R3 when [W] is very low and T is high. Hydration of carboxyl ends becomes important when [W] is large. Based on the mechanism in Table 1 and two reactions (similar to R2 and R3) that occur during nylon 6,6 polymerization, an expression for K_a is developed and fitted using all literature nylon 6 and 6,6 equilibrium data over a wide range of conditions (0.06<[W]<52 molkg~(-1); 220