Melt‐Phase Nylon 612 Polycondensation Kinetics: Effects of Sodium Hypophosphite Catalyst

Abstract

You et al., 1994). This study examines the catalytic effect of sodium hypophosphite (SHP). SHP is added to polyamides because, in addition to its catalytic properties, SHP acts as a whitening agent (Lysek and Ables, 2001), as well as a thermal stabilizer and antioxidant (Agouri and Muller, 1968;Kelmchuk, 1972;Kazuhiko et al., 1995), which gives desirable end-use properties to nylon fi bres and moulded products.SHP catalyzes a variety of esterifi cation and amidation reactions. For example, Fourier Transform Infrared (FTIR) and HighPerformance Liquid Chromatography (HPLC) studies have confi rmed that: SHP catalyzes the formation of anhydrides and water from polycarboxylic acids (Yang et al., 1996;Schramm and Rinderer, 2004), an important intermediate step in the formation of ester linkages between polycarboxylic acids and cellulose; SHP catalyzes esterifi cation reactions between anhydrides and hydroxyl Effects of sodium hypophosphite (SHP) catalyst and water on nylon 612 melt-phase polycondensation reactions were investigated using dynamic experiments with different catalyst levels (33, 113 and 249 ppm). The water concentration in the nylon melt was varied by altering the composition of a steam/nitrogen mixture bubbled through the molten polymer. Carboxyl and amine end-group concentrations were determined for polymer samples collected during the experiments. A model was developed to describe the kinetics of nylon polycondensation using the SHP catalyst in the high-temperature and low water operating region that corresponds to the fi nal stages of industrial melt-phase polyamidation processes.On a étudié les effets d'un catalyseur d'hypophosphite de sodium (SHP) et d'eau sur les réactions de polycondensation en phase fondue du nylon 612 par des expériences dynamiques avec différents niveaux de catalyseur (33, 113 et 249 ppm). On a fait varier la concentration d'eau dans le nylon fondu en modifi ant la composition d'un mélange vapeur/azote que l'on a fait buller dans le polymère fondu. La concentration des terminaisons carboxyle et amine a été déterminée pour des échantillons de polymères collectés lors des expériences. On a mis au point un modèle pour décrire la cinétique de la polycondensation de nylon avec le catalyseur SHP dans la région de fonctionnement à haute température et à faible concentration d'eau qui correspond aux stades fi naux des procédés industriels de polyamidation de la phase fondue.

Improved Kinetic Rate Constants for Nylon 6, Nylon 6,6, and Nylon 6/6,6 Copolymer

“…As shown in Table 7 , they used these data to estimate 20 kinetic parameters (including 10 activation energies) and 10 equilibrium parameters (including 5 reaction enthalpies). Parameter values estimated by other research groups (using smaller data sets or unpublished data) are also shown in Table 7 11,14–16,18,24,25,32,35,42,45–547 are similar for different research groups (e.g., the activation energy estimate of 78.7 kJ mol −1 for R1 reported by Arai et al11 is similar to 78.5 kJ mol −1 reported by Reimschussel et al,45 other parameter estimates differ by orders of magnitude between research groups.…”

“…Parameter values from both Step 1 and Step 2 result in overprediction of the rate of polycondensation at high [ W ] (i.e., for experiments with initial water ≥3.5 wt%) and to underpredict the rate of polycondensation at low [ W ] (e.g., for experiments with initial water ≤1.18 wt%). This result is not particularly surprising because many authors have commented on the complex influence of water on polycondensation equilibrium and reaction rates 14–28. In our previous reaction equilibrium modeling study,28 side reactions involving amidine ends, cyclopentanone ends, and hydrated carboxyl ends (R7–R12 in Table 5) were used to develop a model for the complex behavior of K 2 a , which changes with [ W ] and T .…”

“…In Equation , Mallon and Ray assume that so that polycondensation is always second order with respect to [ C ]. This assumption is questionable for some industrial nylon polymerization reactors that are operated with low [ C ] 11,16,30. For example, Zheng et al showed (using nylon 6,12 experiments) that polycondensation is first order in [ C ], rather than second order, at the low‐water and low‐carboxyl conditions encountered during the final stages of industrial nylon 6,6 production 16.…”

“…This assumption is questionable for some industrial nylon polymerization reactors that are operated with low [ C ] 11,16,30. For example, Zheng et al showed (using nylon 6,12 experiments) that polycondensation is first order in [ C ], rather than second order, at the low‐water and low‐carboxyl conditions encountered during the final stages of industrial nylon 6,6 production 16. Mallon and Ray demonstrated that their kinetic model (and also the model of Steppan et al) provide “reasonable representations” of the behavior of Ogata's nylon 6,6 kinetic data obtained at 200–220 °C with [ W ] 0 = 1.84–22.6 mol kg −1 14…”

“…Simulation results from our nylon 6/6,6 copolymer modeling studies (and nearly all nylon 6 modeling studies in the literature) rely on values of rate constants and equilibrium constants determined by Arai et al, who estimated their parameters using nylon 6 batch reactor data from a large number of experiments at relatively low water concentrations (0.8–2.1 wt%) 11. Due to the limited range of operating conditions, several research groups5,6,14–16 have expressed concerns about using Arai's parameter values over the much wider range of conditions that are typically encountered during nylon 6,6 homopolymerization and nylon 6/6,6 copolymerization. For example, it is well known that Arai's parameters do not account for the complex influence of water on nylon 6 and nylon 6,6 reaction equilibria that has been observed in the literature, especially for polycondensation reaction R2 17–27.…”