Comparison of stabilization by Vitamin E and 2,6-di-tert-butylphenols during polyethylene radio-thermal-oxidation

Highlights

• Radio-thermal oxidation of PE+phenolic antioxidants.

• Comparison of Vitamin E and 2,6-di-tert-butylphenols.

• Kinetic modeling for predicting practical cases.

Abstract

This paper reports a compilation of data for PE+Vitamin E and 2,6-di-tert-butylphenols oxidation in radio-thermal ageing. Data unambiguously show that Vitamin E reacts with P and POO whereas 2,6-di-tert-butyl phenols only react with POO. Kinetic parameters of the stabilization reactions for both kinds of antioxidants were tentatively extracted from phenol depletion curves, and discussed regarding the structure of the stabilizer. They were also used for completing an existing kinetic model used for predicting the stabilization by antioxidants. This one permits to compare the efficiency of stabilizer with dose rate or sample thickness.

Introduction

Unstabilized polyolefins oxidation proceeds by an in chain radical mechanism characterized by an initially long chain kinetic length, meaning that propagation reactions

$$\begin{array}{l}\mathrm{P}{+O}_2\to \mathrm{POO}{k}_2\\ \mathrm{POO}+\text{PH}\to \text{POOH}+P{k}_3\end{array}$$

predominate over termination. Oxidation can be retarded if stabilizers compete with propagation. Due to their O–H bond weaker than polyethylene C–H one, phenols show the requested features:

Despite it is non-reactive, the R3-group influences the physical aspects of stabilization: solubility (Billingham et al., 1991), diffusion (Al-Malaika et al., 1991), and evaporation (Calvert and Billingham, 1979).

2,6-di-tert-butylphenols are the most current family of antioxidants for polyolefins (Schwarzenbach et al., 2001). They react by donating a hydrogen atom to a chain carrying peroxy radical. The resulting phenoxyl radical A isomerizes, and then reacts with another peroxyl or with O₂, with another phenoxyl by dismutation or coupling, or generate a new form of stabilizer (Allen et al., 1985, Pospíšil, 1991, Pospíšil, 1993, Pospíšil et al., 1996, Pospı́šil et al., 2002). These mechanisms can be represented by a “kinetically equivalent” scheme (Richaud et al., 2011, Richaud, 2013) using a limited number of adjustable parameters

$$\begin{array}{l}\mathrm{POO}+\text{AH}\to \text{POOH}+\mathrm{A}{k}_{\mathrm{S1}}\\ \mathrm{POO}+\mathrm{A}\to \mathrm{POO}–\mathrm{A}{k}_{\mathrm{S2}}\end{array}$$

which simulates the main features of stabilization by phenols in polyolefins: linear increase of the induction period with initial phenol concentration, negligible changes of the maximal oxidation rate i.e. steady state characteristics, and stabilizer depletion during thermal oxidation.

Vitamin E (α-tocopherol) is another phenol having anti-inflammatory action (Tahan et al., 2011, Reiter et al., 2007). Its structure.

is close of phenolic antioxidants. However, there are two specificities:

• Methyl substituent in 2 and 6 positions instead of tert-butyl.

• A linear aliphatic chain favoring its solubility in lipids, and a low molar mass increasing its diffusivity.

Vitamin E was shown to stabilize UHMWPE during the post irradiation exposure, with slower build-up of ketones, hydroperoxides (Costa et al., 2009, Mallégol et al., 2001a) and a decrease of the concentration of intermediary unstable radicals (Jahan and Walters, 2011). In the case of squalane oxidation monitored by Oxidation Induction Time (OIT) at 200 °C (Breese et al., 2000), the changes of OIT with phenol concentration were shown to be greater for Vitamin E than for AO1 and AO2 (see Appendix A for structure). The lower molar mass (and so high volatility) of AO2 explains why it is less efficient than AO1. However, Vitamin E is strongly more efficient than AO1 despite its molar mass two times lower than AO1. Al-Malaika and Peng (2008) compared the melt stabilization of a LLDPE with 900 ppm AO3 (ca. 16.1×10⁻⁴ mol l⁻¹ in molten polymer) and 300 ppm Vitamin E (ca. 6.6×10⁻⁴ mol l⁻¹) and observed a very close behavior, suggesting that Vitamin E is more efficient than a hindered phenol of comparable structure even at a lower concentration.

Even if several by-products were evidenced in thermally degraded PE by Al-Malaika et al. (2001), its stabilization mechanism is expected to have some commonality with other 2,6-di-tert-butylphenols (Mallégol et al., 2001b, Lucarini and Pedulli, 2007).

Some authors have compared AO differing by R1, R2 and R3 groups and addressed the influence of electro-attractive effects on the antioxidant properties (Amorati et al., 2006, Amorati et al., 2007). Based on the assumption that Vitamin E and 2,6-di-tert-butylphenols have the same stabilization mechanism, Lucarini and Pedulli (2007) have compiled rate constants for the reaction between POO and phenols and observed that

$$k_{\mathrm{inh}}(\mathrm{\alpha },\mathrm{\beta },\mathrm{\gamma },\mathrm{\delta }\text{tocopherol})⪢k_{\mathrm{inh}}(\text{AO2}).$$

However, they also reported very comparable bond dissociation energies values for the O–H group of phenol, which is in contradiction with the observed difference between rate constants of the reaction towards POO radicals (more than 2 decades). Is it due to the method of radical generation (AIBN initiated oxidation), or the hypothesis made on Vitamin E stabilization, or the method for solving the kinetic scheme of the oxidation (analytical instead of numerical)?

This paper is hence aimed at explaining those results by using the kinetic analysis as a comprehensive tool, and draw conclusions on the effect of ortho substituents on the kinetics of stabilization by

• reviewing literature to derive a mechanistic scheme for Vitamin E stabilization in PE and calculating kinetic parameters under several ratio-thermal conditions.

• comparing the implications of differences between Vitamin E and other sorts of hindered phenols, so as to link the nature of aromatic group substituent with phenol rate constants of stabilization.

• using kinetic model to clarify the radio-thermal-oxidation for PE stabilized with each sort of phenols.