A new kinetic model for predicting polyamide 6-6 hydrolysis and its mechanical embrittlement

Abstract

PA 6-6 hydrolysis at 60, 70, 80 and 90 C in distilled water has been studied by Fourier transform infrared spectroscopy, viscometry in molten state, differential scanning calorimetry and uniaxial tensile testing. The molar mass decreases sharply from the early periods of exposure to reach an equilibrium value of about M nE z 10À11 kg mol À1 almost temperature independent. Hydrolytic chain scissions destroy the entanglement network in the amorphous phase and liberate small macromolecular segments which rearrange locally and initiate a chemicrystallisation. As expected, the embrittlement occurs at a very low conversion of the hydrolysis, in particular when the number average molar mass becomes lower than a critical value of about M nF z 17 kg mol À1 , i.e. very close to its initial value. A new kinetic model has been derived from the classical mechanistic scheme of reversible hydrolysis. This model describes satisfyingly all the kinetic characteristics of the reversible hydrolysis of PA 6-6 not controlled by water diffusion: decrease in molar mass, increase in crystallinity ratio and decrease in ultimate elongation, but also of other types of polyamides previously studied, such as PA 11. Moreover, when it is used as an inverse method, this model gives access to the rate constants of hydrolysis and condensation reactions. It is thus an interesting tool for elucidating structure/rate constant relationships in common families of hydrolysable polymers.

Effect of surface treatment methods on the shear bond strength of auto-polymerized resin to thermoplastic denture base polymer

“…The polymer has significant chain-chain interaction arising from hydrogen bonds between the adjacent amide groups 23. It is generally accepted that the polyamides are strongly resistant to chemical agents due to high degree of crystallinity 222528. Thus, they are hard to react with monomers and resin primers of relining materials.…”

“…The hydrolytic breakdown of amide bonds in strong acids is problematic since it would lead to relative decrease in molecular weight of the polymer, severe destruction, and loss of surface integrity 2228. Moreover, the rate constant of acid hydrolysis depends on acid concentration; in the presence of concentrated acids, the reaction rate is decreased due to reduced water activity 202225. Hocker et al24 showed that the rate of polyamide hydrolysis in solutions containing small weak organic acid was twice that of a water hydrochloric acid solution at the same pH.…”

“…The acidic hydrolysis of polyamide is a bimolecular reaction of oxygen protonized amide with water and the formation of the tetrahedral intermediate 2022242526. Mechanical degradation leads to the formation of the radicals affecting the structure of the amide groups and accelerating hydrolysis 26.…”

“…The hydrolytic degradation of polyamide in different environments has been extensively studied 20222526. Acidic conditions have been observed to facilitate amide hydrolysis.…”

Understanding the durability of advanced fibre-reinforced polymer (FRP) composites for structural applications

“…belong to this category. then, the reverse reaction cannot be neglected and the kinetic model is somewhat more complicated (Jacques et al, 2002;El Mazry et al, 2012), especially when acids are present (Merdas et al, 2003). it can be easily shown that the equilibrium molar mass M We (when t AE •) is given by:…”