Hydrolytic kinetic model predicting embrittlement in thermoplastic elastomers

Abstract

A hydrolytic kinetic model predicting chains scissions of a polyurethane elastomer (TPU) containing an anti-hydrolysis agent (stabilization via carbodiimide) was developed. This model is based on four components: uncatalysed hydrolysis, acid-catalysed hydrolysis, carboxylic acid dissociation and competitive carbodiimide-based deactivation of acid. Protons were considered as the key catalyst responsible for the hydrolysis. Model parameters were determined by fitting experimental data measured on unstabilized and stabilized TPUs, aged in immersion from 40 to 90 C. Scission kinetics were predicted for immersion and 50% relative humidity conditions, from 10 to 100 C. Structure-failure property relationships were also investigated, between molar mass and elongation at break. A master curve was established for elongation at break with molar mass, including both TPUs at four ageing temperatures. By combining predictions for scission kinetics with the molar mass-elongation at break master curve and an embrittlement molar mass as the end-of-life criterion, non-Arrhenian lifetime predictions are proposed for all exposure conditions considered.

Relationships between molar mass and fracture properties of segmented urethane and amide copolymers modified by chemical degradation

“…The effect of temperature is shown in Figure 4 for PEBA immersed in seawater and TPU-ether in air. Temperature influence on TPU-ester hydrolysis can be found in Bardin et al 26 In conclusion, this molar mass decline is observed for all polymers in all exposure conditions. However, the kinetics are greatly affected by the TPE chemical structure as well as the nature and temperature of exposure.…”

“…TPU‐ester shows the fastest degradation, TPU‐ether is the most resistant, and PEBA has an intermediate behavior. In a previous study, 26 we clearly identified that the scission phenomenon (causing the molar mass decrease) observed for TPU‐ester was due to hydrolysis. Here we assume that the same phenomenon occurs in PEBA and TPU‐ether.…”

“…Regarding group sensitivity, studies have shown that urethane hydrolysis is one to two orders of magnitude slower than that of amide and ester 32,33 . Nuclear magnetic resonance analysis on TPU‐ester, presented in Bardin et al, 26 confirmed this observation. These tests confirmed that only the ester groups undergo hydrolysis, while urethane groups remain unaffected on the time range needed to reach the total loss of mechanical properties.…”

“…The normalized elongation was plotted here in order to compare grades with each other. This graph also includes the results obtained for a fourth grade, a TPU‐ester containing an anti‐hydrolysis agent (TPU + stabilizer) for which data are available elsewhere 26 . Even though we previously showed that M n and λ b change kinetics strongly depend on aging conditions, a master curve is highlighted.…”

“…This graph also includes the results obtained for a fourth grade, a TPUester containing an anti-hydrolysis agent (TPU + stabilizer) for which data are available elsewhere. 26 Even though we previously showed that M n and λ b change kinetics strongly depend on aging conditions, a master curve is highlighted. In the TPU-ester case, although the anti-hydrolysis agent delays the molar mass drop, it does not significantly affect the relationship.…”