Effects of End-Group Balance on Melt-Phase Nylon 612 Polycondensation: Experimental Study and Mathematical Model Wei Zheng, † Kim B. McAuley,* E. Keith Marchildon, † and K. Zhen Yao Department of Chemical Engineering, Queen’s University, Kingston, Ontario, Canada K7L 3N6 The effects of end-group balance and moisture level on melt-phase polycondensation reactions were investigated using nylon 612. The polycondensation reaction was determined to be first- order in amine ends and first-order in carboxyl ends at the relatively high temperature (284 °C) and low water concentration conditions (0-0.002 mass fraction) studied, which are encountered in the later stages of nylon polymerization processes. Using data from this study and the nonisothermal data of Schaffer et al. (Chemical Pathways and Kinetics of the Later Stages of Nylon Polymerization Processes. Ph.D. Thesis, Queen’s University, Kingston, Ontario, Canada, 2003; Experimental Study and Modeling of Nylon Polycondensation in the Melt Phase. Ind. Eng. Chem. Res. 2003, 42, 2946), a mathematical model was developed that can accurately describe changes in both the polyamidation reaction rate and the apparent equilibrium constant, with changing water concentration and temperature. Introduction Nylons are important commercial polymers that are produced through reversible polycondensation reactions between amine and carboxyl end groups. The forward reaction is a polyamidation reaction, and the reverse reaction is a hydrolysis reaction: Published research on the kinetics of the previous reaction is somewhat contradictory. Some researchers contend that the reaction obeys second-order kinetics (first order in both amine and acid end groups). 3-8 Several studies of polyamidation, under conditions where the end-group concentrations are relatively low (i.e., conversions above 90%), indicate that a carboxyl- catalyzed third-order reaction assumes increasing impor- tance and becomes predominant. So, other researchers 9-13 have suggested that there is a shift from second- to third-order kinetics, involving catalysis by the carboxylic acid end group, at low water contents and high conver- sions. In addition, a study of the hydrolysis of amides at 220 °C in near-neutral buffered solutions has shown that this reaction is carboxyl-catalyzed. Therefore, some researchers have assumed third-order kinetics (first order in amine ends and second order in carboxyl ends) over the entire water concentration range. 14 The majority of experimental studies on nylon kinet- ics and equilibrium in the open literature focus on nylon 6 3,15-24 and nylon 66. 25,26 Both nylon 6 and nylon 66 are commercially important nylon products, but they are not ideal materials for studying polycondensation kinetics. During nylon 6 polymerization, ring opening of capro- lactam and polyaddition reactions occur in addition to the polycondensation reaction, complicating the study of amidation kinetics. In the production of nylon 66, thermal degradation reactions, resulting from the cy- clization of adipic acid segments, 27-30 influence the concentrations of end groups, thereby preventing ac- curate estimation of the rate and equilibrium constants for polycondensation. In this study, nylon 612, a commercial polymer made from dodecanedioc acid (DDDA) and hexamethylenedi- amine (HMD), is used to study nylon polyamidation kinetics. Nylon 612 is more thermally stable than nylon 66 because it does not have adipic acid segments in the polymer chains. Unlike nylon 6, ring-opening and poly- addition reactions do not occur. As in previous studies, we assume that the lengths of the aliphatic portions of the polymer repeat units do not affect the end-group reactivity, 5-8,31,32 so that the kinetics of the polyconden- sation reactions from nylon 612 are comparable with the kinetics for other aliphatic polyamides. Most of experimental studies on nylon kinetics and equilibrium in the open literature focus on conditions of high water content and low temperature, which are encountered early in the polyamidation process; 3,6,26 relatively few data have been reported for conditions of low water content and high temperature, 1,2,25 which are present in the final stages of industrial nylon production processes. A better understanding of this stage is very important for industrial reactor design, product quality control, and optimization of process operating condi- tions. The objectives of this study are to use nylon 612 to examine the effects of end-group balance on the kinetics of the polycondensation reaction, to distinguish whether the polycondensation reaction obeys second- or third-order kinetics in the later stages of polyamide production, and to estimate the rate and equilibrium constants of the polycondensation reaction at high temperatures and low water contents. Experiments Figure 1 shows a schematic diagram of the batch reactor system. The reactor itself is based on the design of finisher reactors described in several patents 33-36 that are used industrially as the final stage of continuous nylon 66 polymerization processes. The reactor vessel is a 5-L stainless steel stirred tank, with two inter- meshing helical impellers entering from the bottom. The * To whom correspondence should be addressed. † Current address: Research and Business Develop Centre, DuPont Canada Inc., Kingston, Ontario, Canada K7L 5A5. -COOH +-H 2 N h -CONH-+ H 2 O 2675 Ind. Eng. Chem. Res. 2005, 44, 2675-2686 10.1021/ie049474n CCC: $30.25 © 2005 American Chemical Society Published on Web 12/17/2004