# Efficient Catalysis by Sodium Hypophosphite for Solid-State Polymerization of High-Viscosity PA6/66 Copolyamide: Kinetics, Process Optimization, and Industrial Application

## TL;DR

This paper introduces sodium hypophosphite as an efficient catalyst for producing high-viscosity PA6/66 copolyamide through solid-state polymerization, improving industrial processes.

## Contribution

The novel use of sodium hypophosphite as a catalyst in solid-state polymerization of PA6/66 is introduced, enabling efficient and stable production.

## Key findings

- Sodium hypophosphite increases the viscosity of PA6/66 linearly during solid-state polymerization.
- Optimal catalyst concentrations (25 ppm at 150°C and 30 ppm at 160°C) maintain stable viscosity increase rates.
- The catalyst was successfully applied in a 4500 tons/year industrial test line for uniform high-viscosity PA6/66 production.

## Abstract

The industrial preparation via solid-state polymerization (SSP) of high-viscosity copolyamides 6/66 (PA6/66) addresses the challenges, including prolonged reaction times, high energy consumption, and uneven viscosity distribution. In this study, sodium hypophosphite was introduced into the PA6/66 copolymerization system as a solid-state polymerization catalyst. The effects of this catalyst on the solid-state viscosity-increasing rate and relative viscosity were systematically investigated, and the extraction process was optimized to solve the loss of catalyst and controllable extractable content. The results showed that the relative viscosity of PA6/66 increased linearly with the SSP time, and the apparent viscosity increase rate could be stably maintained at 0.14 h−1 at 160 °C due to the catalytic action. Based on the phosphorus (P) content in the chips, the viscosity increase rate is not further large when the P content is 25 ppm at 150 °C and 30 ppm at 160 °C, which can be added as a “control concentration” as a catalyst. The extraction kinetics showed that the catalyst concentration of the chip could be kept higher than the control concentration, and the extractable content can satisfy the requirements for processing. The catalyst of sodium hypophosphite was utilized on the 4500 tons/year PA6/66 continuous polymerization test line, and the high-viscosity PA6/66 chips with uniform viscosity were stably prepared. This study provides a reliable theoretical basis and process route for the large-scale continuous preparation of high-quality and high-viscosity PA6/66 resin.