The dissolution

crystallization and hydrolysis behaviors of polyamide 6 (PA 6) in superheated water (140 ℃≤

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 ℃≤

T

H

≤ 155 °C

(III) 155 ℃

<

T

H

<

160 ℃and (IV)

T

H

≥ 160 °C. Below 140 °C

the hydrothermal processing does not have obvious impact on PA 6. Between 140 ℃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 ℃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.