Thermal stabilization of polyamide 11 by phenolic antioxidants

Highlights

• Thermal oxidation of thin PA11 films stabilized with several concentrations of phenolic antioxidants.

• Multiscale study: carbonyl build up, yellowing, molar mass changes, Oxidation Induction Temperature.

• Non Arrhenian behavior for all materials.

• Phenols efficiently protect polymer in terms of molar mass decrease despite a detrimental effect on aspect properties.

Abstract

This paper addresses the effect of hindered phenols (mainly Irganox 1098 with a few comparisons with other phenolic antioxidants) on the stabilization of polyamide 11 aged at several temperatures (90–165 °C). The effect of several phenol concentrations (up to about 0.4%) on kinetic curves for imide build-up, yellowing, and molar mass changes (in association with embrittlement) was investigated. Phenols significantly contribute to yellowing, even at low imide concentrations. When they are used at high concentrations, a post-polycondensation reaction becomes predominant at earlier exposure times, thus increasing molar mass and significantly delaying embrittlement.

Introduction

Polyamides (PAs) are a family of technical thermoplastic polymers that cover a wide range of temperatures of use. Their semi-crystalline morphology along with the presence of polar groups renders them quite resistant to various chemicals, particularly hydrocarbons [[1], [2], [3]]. They are widespread in several industrial applications (e.g., flexible offshore pipes, textiles, sporting gear). In the case of PA11, the end-of-life criteria (from a mechanical point of view) were previously determined and associated with critical values of molar mass and crystalline ratios [4]. The critical molar mass can be easily predicted from kinetic models, at least in the case of unstabilized PA, while changes in the crystallinity ratio can be determined from the chemicrystallization yield [[5], [6]].

We will now describe the effect of antioxidants at both the molecular and macromolecular levels and then establish the kinetic models adapted to the case of stabilized PAs. Previously published papers illustrate well the effect of copper salts [[7], [8], [9]] or phenolic stabilizers on thin PA6 films [7,10,11], for example, when the oxidation is not controlled by oxygen diffusion. In the case of thick samples, antioxidants were shown to change the shape and thickness of oxidation profiles [9] and possibly the subsequent embrittlement of the polymer. The physical parameters describing the action of Irganox 1098 were also estimated [12]. They are valuable inputs for predicting the lifetime of stabilized polymers using semi-empirical [[13], [14], [15]] or physical models that take into account the reactivity of polymers and their stabilizers [[16], [17], [18]].

In this work, we will focus on the case of PA11, including phenolic stabilizers. We will observe the main phenomena associated with stabilizers through a multiscale analysis aimed at tracking the residual antioxidant (using thermal analysis [19,20]), the stable carbonyl products [21], the macromolecular changes (linked to chain scission, crosslinking, and post-polycondensation [4]), and the effect of oxidation on aesthetical properties, namely yellowing (UV spectroscopy [21,22]). For this purpose, we investigated the case of PA11 stabilized with Irganox 1098 (a common antioxidant in polyamides) at various initial concentrations and then made comparisons (at a given concentration) with other phenolic antioxidants.