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

Abstract

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.

Effect of nanoparticle and glass fiber on the hydrothermal aging of polyamide 6

“…[ 24 ] The hydrolysis of PA6 in water leads to the decreases of molecular weight and viscosity without changing the polydispersity. [ 4,25 ] The toughness will also decrease after hydrolysis occurs in PA6. [ 26 ]…”

Structure and properties of bio‐based polyamide 109 treated with superheated water

“…This revealed again that the elevation in the melting temperature could be attributed to an increase in the thickness of lamellae, both of which may be influenced by the annealing effect in superheated water. Moreover, the scattering peak disappeared at 168 °C which was attributed to the occurrence of no order for lamellae packing under this hydrothermal treatment temperature . An increase in reaction temperature resulted in an increase in the intensity of the scattering peak, which reflected that the distribution of the long period was more uniform because the long period is an average value.…”

“…It has higher concentrations of H + and OH − ions compared to ambient water, which is beneficial to ionic, free radical, polar nonionic, hydrolysis, hydration and hydrogen exchange reactions . Based on previous studies, the hydrothermal treatment caused a shift of the molecular weight and polydispersity index to lower values, while the oligomers still had a much higher molar mass than monomers or small‐molecule products . These results were attributed to shortening of the polyamide chains by dissolution‐induced and hydrolysis‐induced scission instead of thermal decomposition scission.…”

“…The relative viscosity ( η r ) was measured at 25 ± 0.1 °C with an Ubbelohde viscometer ( Φ = 1.0–1.1 mm); the measurement was conducted in 98% sulfuric acid solution with 5 mg mL −1 concentration according to Chinese Standard GB12006.1‐89. The relative viscosity ( η r ) is calculated from …”

“…Wang et al demonstrated that PA6 could be dissolved completely when the reaction temperature and time were 160 °C and 24 h respectively, while PA6 could be hydrolyzed continuously owing to an increase in reaction temperature and time. The surface of PA6 film was also modified to improve adhesion and printing performance after hydrothermal treatment . The hydrothermal technique provided significant advantages for the recycling or processing of PA6 granules and modifying the surface structure of PA6 films.…”