Effect of Diacid Stabilizers on Kinetics of Hydrolytic Polymerization of e-Caprolactam in Industrial Reactors Ashwini K. Agrawal, 1 Deepak K. Gupta, 1 Kotilingam Devika, 1 Tomonobu Manabe 2 * 1 Department of Textile Technology, Indian Institute of Technology, New Delhi 110 016, India 2 Modipon Fibers Company, Modinagar 201 204, Uttar Pradesh, India Received 31 October 2005; accepted 22 March 2006 DOI 10.1002/app.24558 Published online 5 February 2007 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: The rate constants in hydrolytic polymeriza- tion of e-caprolactam are dependent on the concentration of carboxylic acid groups in the reaction medium. Therefore, the use of diacid stabilizers for regulating molecular weight are likely to have favorable effect on the kinetics of polymer- ization compared to monoacid stabilizers, which are tradi- tionally used in such polymerizations. To understand the kinetics of polymerization in the presence of diacid stabilizer compared to monoacid stabilizer, mathematical kinetic mod- els were developed using the end group approach. These models were used to quantify the effect of both stabilizers on nylon-6 synthesis in a closed isothermal batch reactor at different temperatures in the range of 245–2658C. The kinetic model for the diacid-stabilized system was then extended to an industrial VK tube reactor using the process model devel- oped earlier for the monoacid stabilized system. Both the mathematical modeling and experimental results showed that the presence of diacid stabilizer could significantly enhance the overall kinetics of the reaction compared to the monoacid stabilized system and in turn, resulted in reduc- tion of the polymerization time by about 20–25%. The study suggests that diacid stabilizers may be used preferably over monoacid stabilizers in synthesis of nylon-6 to reduce the cost of polymerization. Ó 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2065–2075, 2007 Key words: polymerization kinetics; nylon-6; VK tube; diacid stabilizer; modeling INTRODUCTION Nylon-6 is losing its market share to other commodity polymers such as polypropylene and polyester because of the higher cost of its raw materials. Solution to this prob- lem is to use efficient processes or introduce additional functionality to the polymer to realize better premium on nylon products in the market. To achieve this goal it is im- portant to understand basic reaction kinetics and polymer- ization in industrial reactors. Kinetics of hydrolytic poly- merization of e-caprolactam has been extensively stud- ied 1–10 in batch reactors. These kinetic models were also used to investigate the polymerization process in continu- ous tube reactors. 5,11–19 Recently, we had reported model- ing of an industrial VK (Vereinfacht Konitnuierliches) tube reactor, using monocarboxylic acid (monoacid) stabi- lizer, where the effect of the complex internal structure (heat exchanger, internal gratings) of the reactor was con- sidered. 20 The effect of hydrostatic pressure inside the re- actor was taken into account to predict water profile through out the reactor. Using temperature and pressure profile, lowest water content (LWC) point was predicted, which was found to be critical in determining the proper- ties of the end product. The model was found to be very useful in controlling and optimizing the performance of an industrial VK tube reactor. In the aforementioned studies, although most of the reaction parameters that affect kinetics of the nylon-6 synthesis, such as temperature, water concentration, and monoacid stabilizer concentration, have been con- sidered, the effect of the type of stabilizer on reaction kinetics has not been investigated. It is reported 3 that the carboxylic functional groups act as a catalyst in the hydrolytic polymerization of e- caprolactam, and therefore, the rate constants for all reactions (k i ) in hydrolytic polymerization of e-caprolac- tam can be shown to depend on the concentration of the [COOH] groups available at any point in the reaction system. This effect has been quantified by the following empirical relation: 2–6,12,16,21 Rate constant ðk i Þ¼ k i 0 þ k i c ½COOH (1) In synthesis of nylon-6, apart from monoacid stabil- izers, dicarboxylic acid (diacid) stabilizers can also be used to regulate the molecular weight of the polymer. In recent years, diacid stabilizers are gaining impor- tance in this polymerization because they can be read- *Present address: Manufacturing Technology Consultant, 545 White Oak Road, Blue Bell, Pennsylvania 19422. Correspondence to: A. Agrawal (ashwini_agrawal@yahoo. com). Contract grant sponsors: Ministry of Human Resource Development, Govt. of India; Modipon Fibers Company, Modinagar, U.P., India. Journal of Applied Polymer Science, Vol. 104, 2065–2075 (2007) V V C 2007 Wiley Periodicals, Inc.