Modeling polycondensation equilibrium for Nylon 6 and Nylon 6,6

Conference Dates

May 20-25, 2018


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 Ka 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].[1] Also, Ka decreases with increasing T. This complex behaviour of Ka has long been a topic of mystery and practical concern because reliable values of Ka are important for simulating and optimizing nylon reactor systems.[1-3] 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.[4,5] Hydration of carboxyl ends becomes important when [W] is large.[3] Based on the mechanism in Table 1 and two reactions (similar to R2 and R3) that occur during nylon 6,6 polymerization,[6,7] an expression for Ka 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£T£300 °C).[1,2] The resulting expression (used to obtain the curves in Figure 1) was developed by recognizing that experimental values of Ka are obtained using titrations where basic amidine ends are counted as if they are amine ends and hydrated carboxyl ends are counted as if they are regular carboxyl ends, so that Ka = [L][W]/{([C]+[C*])([A]+[I])}. A good fit was obtained for all 144 data points used to estimate the five parameters that appear in the final expression (i.e., values of K1, K2 and K4 at a reference temperature of 240 °C and reaction enthalpies for R1 and R2). The resulting relatively simple equation will be used in ongoing model development for nylon 6/6,6 copolymerization.

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