Comparison of stabilization by Vitamin E and 2,6-di-tert-butylphenols during polyethylene radio-thermal-oxidation
Introduction
Unstabilized polyolefins oxidation proceeds by an in chain radical mechanism characterized by an initially long chain kinetic length, meaning that propagation reactionspredominate 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 parameterswhich 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:
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Methyl substituent in 2 and 6 positions instead of tert-butyl.
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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
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
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reviewing literature to derive a mechanistic scheme for Vitamin E stabilization in PE and calculating kinetic parameters under several ratio-thermal conditions.
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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.
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using kinetic model to clarify the radio-thermal-oxidation for PE stabilized with each sort of phenols.
Section snippets
Kinetic modeling of stabilization by Vitamin E
According to the mechanism proposed in Mallégol et al. (2001a–c) and Lucarini and Pedulli (2007) and used in Amorati et al., 2006, Amorati et al., 2007, Vitamin E would not react when polymer is aged in inert atmosphere, typically during the UHMWPE sterilization by γ radiation. However, according to Costa et al. (2009) according to which Vitamin E is consumed when polymer is irradiated under nitrogen. The initial rate of stabilizer depletion is ca. 2.3×10−4 mol l−1 kGy−1 under inert atmosphere
Effect of substituent on stabilization rate constants
Mallégol et al. (2001c) reported depletion kinetics of Vitamin E and other 2,6-di-tert-butylphenols under irradiation, which offers another possibility to determine the stabilization rate constants from inverse method. Given the possible sources of errors (such as the presence of residual phosphites leading to a strong synergy with phenols, irradiation induced temperature rise, crystallinity profile in the thickness modifying initial stabilizer repartition and diffusivity of oxygen and
Effect of radiochemical initiation on phenol efficiency
In their original work, Clough and Gillen (1990) already illustrated the efficiency of a phenol stabilizer at 200 kGy h−1 dose rate. Here, the kinetic model permits in principle to estimate this efficiency in a wider range of exposure conditions.
The sets of rate constants (Table 1) used for simulating Fig. 7 were kept for running simulations differing by the dose rate value. This set of simulations is aimed at understanding the changes of stabilizer efficiency for inhibiting degradation phenomena
Conclusions
Most of the literature on polyolefin oxidation considers that phenols trap POO radicals. From a kinetic point of view, a stabilization effect is observed if
Hence, phenols are actually expected to be efficient stabilizers since
Since BDE(O–H) is almost the same in Vitamin E and in 2,6-di-tert-butylphenols, both sort of antioxidant should have a very comparable efficiency in PE. However, a review of literature shows that Vitamin E is
Acknowledgments
Science et Médecine company (Créteil – France) is gratefully acknowledged for having funded this research work. Dr. Frédéric Bréard is acknowledged for fruitful talks.
References (47)
- et al.
The photo-stabilisation of polypropylene: a review
Polym. Degrad. Stab.
(1985) - et al.
Migration of 4-substituted 2-hydroxy benzophenones in low density polyethylene: part I—diffusion characteristics
Polym. Degrad. Stab.
(1991) - et al.
The antioxidant role of Vitamin E in polymers V. Separation of stereoisomers and characterisation of other oxidation products of dl-α-tocopherol formed in polyolefins during melt processing
Polym. Degrad. Stab.
(2001) - et al.
Stabilisation of metallocene ethylene-1-octene copolymers during multiple extrusions
Polym. Degrad. Stab.
(2008) - et al.
The solubility of stabilizing additives in polypropylene
Polym. Degrad. Stab.
(1991) - et al.
Stabilisation of ultra-high molecular weight polyethylene with Vitamin E
Polym. Degrad. Stab.
(2007) - et al.
Improving synthetic hindered phenol antioxidants: learning from Vitamin E
Polym. Degrad. Stab.
(2000) - et al.
Stabilizer additives in ionizing radiation environments under oxidizing conditions
Polym. Degrad. Stab.
(1990) - et al.
Post electron-beam irradiation oxidation of orthopaedic UHMWPE
Polym. Degrad. Stab.
(2008) - et al.
Post electron-beam irradiation oxidation of orthopaedic Ultra-High Molecular Weight Polyethylene (UHMWPE) stabilized with Vitamin E
Polym. Degrad. Stab.
(2009)
High temperature melted, radiation cross-linked, Vitamin E stabilized oxidation resistant UHMWPE with low wear and high impact strength
Polymer
Occurence and implications of radiation dose-rate effects for material aging studies
Rad. Phys. Chem.
Rigorous experimental confirmation of a theoretical model for diffusion-limited oxidation
Polymer
Macroradical reaction in ultra-high molecular weight polyethylene in the presence of Vitamin E
Rad. Phys. Chem.
A simplified approach for the lifetime prediction of PE in nuclear environments
Nucl. Instrum. Methods B
Oxidation of polyethylene under irradiation at low temperature and low dose rate. Part II. Low temperature thermal oxidation
Polym. Degrad. Stab.
The kinetics of oxidative induction of LDPE stabilized with commercial antioxidants
Polym. Degrad. Stab.
Post-γ-irradiation reactions in Vitamin E stabilised and unstabilised HDPE
Nucl. Instrum. Methods B
Antioxidant effectiveness of Vitamin E in HDPE and tetradecane at 32 °C
Polym. Degrad. Stab.
A comparison of phenolic antioxidant performance in HDPE at 32–80 °C
Polym. Degrad. Stab.
Diffusion des additifs du polyethylene—I: influence de la nature du diffusant
Eur. Polym. J.
Effectiveness of antioxidants: suppression of evolution of gaseous degradation products from low-density polyethylene during thermo-oxidation
Polym. Degrad. Stab.
Characterization of irradiated blends of image-tocopherol and UHMWPE
Biomaterials
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