Gelation Behavior of Retained MX Nylon in an Extruder

Polyamide MXD6 is used as a packaging material for films and PET bottles because of its excellent gas barrier properties. One of the requirements for MXD6 is the suppression of gelation during film manufacturing because it leads to defective appearance and unstable production. To elucidate the gelation mechanism, heat deterioration behavior of retained MXD6 in an extruder was investigated. This paper reports the investigation results and shows the presumed gelation mechanism of MXD6 and anti-gelation mechanism based on the addition of sodium acetate and PA6. Furthermore, evaluation results of anti-gelation formula by a new evaluation method were reported. As a result of various investigations, it was estimated that MXD6 gelation proceeds in four steps. At first, MXD6 undergoes hydrolysis during heat retention to form amino and carboxy terminals. After that, generated carboxy terminal is eliminated as a cyclopentanone causing rapid increase of amino terminal. Then the reaction between these amino terminals forms secondary amines. Finally, they react with the carboxyl end group to generate tertiary amides presumed to be a cause of MXD6 gel. Addition of sodium acetate and PA6 to MXD6 suppresses the gelation. Our investigation results suggested that this is probably due to the suppression of tertiary amides generation through the action of sodium acetate and PA6 as a base or through cyclization of the PA6. To demonstrate effectiveness of anti-gelation formula, new gelation test method using an extruder was devised. In contrast to the conventional simplified method (heat-press), this method accurately reproduces the retention in an extruder. As a result, addition of sodium acetate and PA6 performed a gelation inhibit effect even in this test. From this results, anti-gelation formula is expected to be useful in film manufacturing. Also, both tests show almost the same results, indicating that both test methods are useful for evaluating gelation behavior.