Dissolution, hydrolysis and crystallization behavior of polyamide 6 in superheated water

The dissolution, crystallization and hydrolysis behaviors of polyamide 6 (PA 6) in superheated water (140 °C ≤ T H ≤ 200 °C) are investigated. The hydrothermal processing of PA 6 can be divided into four regions: (I) T H < 140 °C, (II) 140 °C ≤ T H ≤ 155 °C, (III) 155 °C < T H < 160 °C and (IV) T H ≥ 160 °C. Below 140 °C, the hydrothermal processing does not have obvious impact on PA 6. Between 140 °C and 155 °C, an annealing effect is observed that the hydrothermally treated resin shows increased melting temperature and lamellar thickness compared with the original PA 6. Between 155 °C and 160 °C, the hydrothermal processing induces both annealing and surface swelling. Above 160 °C, PA 6 dissolves fully in the superheated water. As PA 6 dissolves in the superheated water, hydrolysis takes place and becomes more prominent at higher temperatures and longer processing time. The hydrolysis induced molecular weight decrease fits an exponential decay.