Radiochemical ageing of ethylene–propylene–diene elastomers. 4. Evaluation of some anti-oxidants

doi:10.1016/j.polymdegradstab.2005.04.018

Polymer Degradation and Stability

Volume 91, Issue 1, January 2006, Pages 136-143

https://doi.org/10.1016/j.polymdegradstab.2005.04.018 Get rights and content

Abstract

The preceding papers of this series were devoted to the identification and quantification of the main chemical changes resulting from the radiochemical ageing of EPDM (77.9% ethylene, 21.4% propylene, 0.7% diene) and EPR (76.6% ethylene, 23.4% propylene) films irradiated under oxygen atmosphere using ⁶⁰Co γ-rays. It was shown that two processes are involved in the EPDM radio-oxidation. The random γ-radiolysis of the polymer provides a constant source of macro-alkyl radicals that are likely to initiate a selective oxidation of the polymer through free-radicals reactions involving the abstraction of labile hydrogen atoms. In the present paper, infrared spectroscopy has been used to study the γ-degradation of EPDM cross-linked with dicumyl peroxide and/or stabilised with two types of anti-oxidants (hindered phenol or amine-type). The results show that the anti-oxidants are not efficient in preventing oxidation. To understand the lack of efficiency of the stabilisers, the impacts of the various formulations on the rate of degradation of EPDM against chain oxidation involved in thermal and UV ageing were also studied.

Introduction

The modifications of the structure and properties of polymers resulting from exposure to ionising radiation has attracted much interest. The present study, developed in a series of articles, concerns the radiative ageing of EPDM copolymers which are composed of ethylene (PE), propylene (PP) and diene monomer (5-ethylidene-2-norbornene (ENB)). The aim is to propose mechanisms accounting for the main routes of EPDM degradation. Irradiations were carried out both in the presence and in the absence of oxygen, and the comparison with EPR containing the same ethylene/propylene ratio than EPDM were made to assess the importance of the diene moiety and the involvement of oxygen.

The first paper focused on the modifications of the chemical structure of EPDM (77.9% ethylene, 21.4% propylene, 0.7% diene) and EPR (76.6% ethylene, 23.4% propylene) under argon atmosphere [1]. Irradiation of EPDM and EPR was shown to generate trans-vinylene, vinyl, vinylidene and diene unsaturation, with similar radiochemical yields for both polymers. Degradation was shown to also involve cross-linking and production of hydrogen. The presence of the diene, the double bonds of which are consumed with a high radiochemical yield, contributes to the increase in rate and intermolecular bridge density. Mechanisms were proposed to account for the main routes of EPDM degradation. In particular, it was shown that the depletion of unsaturation, which cannot result from unselective direct radiolysis, involves addition by macro-radicals formed in the EPDM radiolysis.

In paper 2 [2], the chemical changes occurring in EPDM and EPR films γ-irradiated under oxygen atmosphere were identified and quantified using a combination of IR analysis, derivatisation reactions and iodometric titration. The extent of cross-linking was evaluated by gel fraction methods.

On the basis of these experimental results, mechanisms accounting for EPDM γ-degradation under oxygen atmosphere were proposed in paper 3 [3]. Discussion took also into account complementary experimental results showing the instability of hydroperoxide groups under γ-radiation. It was shown that radio-oxidation involves two concomitant processes. As the interaction of high energy radiation with matter is unselective, the irradiation of EPDM (77.9% ethylene) mainly involves C–H bond scissions of the ethylene units. Radicals resulting from the direct effects of radiation are likely to initiate a chain oxidation process. It was shown that the methylene groups in an α-position to the double bonds of the ENB are the most oxidisable site and that saturation reactions of the ENB moieties cause the cross-linking of EPDM films.

The present paper reports some recent results and obtained ion stabilisation of EPDM to radiochemical ageing. The EPDM samples were vulcanised with dicumyl peroxide and/or stabilised with hindered phenol or amine-type anti-oxidant. In order to verify the efficiency of the stabilisers against chain oxidation, the impact of the various formulations on the oxidation rate of EPDM induced by UV-light or by temperature was also studied.

Thermo-oxidation, photo-oxidation and γ-irradiation involve a chain-oxidation process whose common points can be described according to [4]:

The main differences between the processes lie in the initiation step and in the hydroperoxide decomposition. It is generally admitted that photo- and thermo-oxidation starts on the ENB moiety before reaching the ethylene–propylene units [5], [6]. The thermal and photochemical degradation of hydroperoxides involves the scission of the O–O bonds [7].

It is briefly recalled that anti-oxidants (AH) prevent by chain transfer (i) or termination (ii) the propagation reactions [7] in any oxidation process (UV, thermal or radiation initiated), according to:

The present paper reports a study of the behaviour of various formulations under conditions of oxidation initiated by UV-light, temperature or γ-radiations. The objective is to compare the efficiency of the anti-oxidant in stabilising EPDM against these various conditions of degradation.

Section snippets

Experimental

Ethylene–propylene–diene monomer (EPDM), based on 5-ethylidene-2-norbornene (ENB), was supplied by Dupont under the trade name NORDEL IP 3725. The molar composition of EPDM determined by solid state ¹³C NMR was 77.9% ethylene, 21.4% propylene, 0.7% diene. Various formulations of EPDM were processed: a pure gum (EPDM), an elastomer cross-linked agent with dicumyl peroxide (Aldrich) and an elastomer stabilised with two types of anti-oxidants: a hindered phenol (Irganox 1035, Ciba-Geigy) and an

Radio-oxidation

Radio-oxidation of EPDM leads to notable modifications in the IR and UV–vis spectra of the samples. The oxidation products formed by γ-irradiation of additive-free EPDM 9 were characterised by IR spectroscopy. The results obtained are shown in Fig. 1: the development of a broad band in the hydroxyl absorption region centred at 3400 cm⁻¹ is observed and in the carbonyl region the formation of a carbonyl band with an absorption maximum at 1714 cm⁻¹.

Focussing on these two regions of the FTIR region

Discussion

Oxidation of polymers induced by UV-light, temperature or γ-radiation results in chain scission, cross-linking and formation of oxygen-containing functional groups. The stabilisers role is to retard these oxidation processes. Oxidation prevention involves the scavenging of free-radical intermediates (P radical dot and PO₂ radicals) and in some cases can also involve in the reduction of radicals formation.

Conventional anti-oxidants are highly efficient against EPDM thermal oxidation and provoke long

Conclusion

The oxidation rate of EPDM can be efficiently reduced by anti-oxidants in thermo-oxidation conditions. In photo-oxidation conditions, anti-oxidants can induce the EPDM degradation because they are not protected by UV-absorbers. In γ-irradiation conditions, anti-oxidants are rapidly consumed and only the amine-type formulation at 1% is observed to display a limited stability, with a very short induction period (25 kGy) before the oxidation starts.

A more effective stabilisation system should

References (23)

A. Rivaton
Polymer Degradation and Stability
(1995)
M. Piton et al.
Polymer Degradation and Stability
(1996)
J.L. Gardette et al.
Polymer Photochemistry
(1986)
R.F. Becker et al.
Polymer Degradation and Stability
(1988)
S. Falicki et al.
Polymer Degradation and Stability
(1994)
J. Mallegol et al.
Nuclear Instruments and Methods in Physical Research B
(2001)
A. Rivaton et al.
Nuclear Instruments and Methods in Physical Research B
(2004)
Y.S. Soebianto et al.
Polymer Degradation and Stability
(1993)
A. Rivaton et al.
J Polymer Science Part A: Polymer Chemistry
(2004)
A. Rivaton et al.
Nuclear Instruments and Methods in Physical Research B
(2004)

A. Rivaton et al.

Nuclear Instruments and Methods in Physical Research B

(2004)

Cited by (58)

The contribution of BaTiO<inf>3</inf> to the stability improvement of ethylene-propylene-diene rubber. Part III. – Comparative assay: EPDM vs EPR
2024, Radiation Physics and Chemistry
This study presents the functional effects of doped BaTiO₃ powders loaded in ethylene-propylene elastomers (copolymer – EPR and terpolymer – EPDM) investigated by chemiluminescence (CL). The presence of foreign atoms (lanthanum and cerium) in the barium titanate lattice produces an improved activity on the stabilization of polymer matrices. The concentration of filler was 1 wt%, where the doping atoms behave as the trapping radicals assuring the increase of the low onset oxidation temperatures for pristine material (168 °C for EPDM and 175 °C for EPR) to higher values for BaTiO₃/La (183 °C for EPDM and 189 °C for EPR) and BaTiO₃/La (190 °C for EPDM and 195 °C for EPR) after receiving 25 kGy. The isothermal determinations point out the protection efficiency proved by the lower positions of the recorded CL spectra for composites in respect with pristine polymers. The compared protection activities at 100 kGy exhibited by the studied composites highlight the greater contribution of BaTiO₃/Ce than BaTiO₃/La indicating the dependence of stabilization effectiveness on the nature of dopant. The present results state the structural influence of backbone configuration on the filler effects on the amelioration of thermal and radiation strengths.
Synthesis and antioxidation behavior in EPDM of novel macromolecular antioxidants with crosslinking and antioxidation effects
2022, Polymer Degradation and Stability
A novel antioxidant denoted as HVA-2@MB was synthesized through the reaction of N,N'-1,3-phenylene bismaleimide (HVA-2) and 2-mercaptobenzimidazole (MB) catalyzed by triethylamine. This antioxidant had high molecular weight as well as crosslinking and antioxidation functions. Then, the effect of HVA-2@MB on the crosslinking and antioxidation behaviors of ethylene-propylene-diene rubber (EPDM) was investigated. HVA-2@MB greatly reduced the inhibitory effect of antioxidants on rubber vulcanization and improved the crosslinking density of rubber. In addition, due to the reactivity of HVA-2@MB, the antioxidant groups were chemically bonded to the rubber network, which significantly reduced the migration of antioxidants in the thermo-oxidative environment, thereby improving the thermo-oxidative stability of the rubber. Therefore, compared with MB/EPDM composites, HVA-2@MB/EPDM composites exhibited better mechanical properties and thermo-oxidative stability during thermo-oxidative aging.
Availability of PLA/SIS blends for packaging and medical applications.Part II: Contribution of stabilizer agents
2022, Radiation Physics and Chemistry
The extending material durability of poly(lactic acid) (PLA)/styrene-isoprene-styrene copolymer (SIS) blend with 30% of SIS by the presence of acrylic acid (AA) and two stabilizers, caffeic acid or vanillic acid, is evaluated by γ-irradiation at technological doses (25 or 50 kGy) applied for sterilization or for the manufacturing of several commodities, medical, and packaging items, respectively, where an advanced exposure would affect the post-irradiation oxidation stability. The chemiluminescence investigations provide the onset oxidation temperature values from which the activation energies of degradation were calculated. Differential scanning calorimetry and infrared spectroscopy complete the evaluation of improvement efficiencies that characterize the thermal behavior of these materials. The effects of γ-exposure on the material properties are discussed relative to the radiolysis modifications occurred during radiation processing. The relevance of oxidation protectors is analyzed based on the efficient delay of ageing. The results are the qualification information for the packaging and medical products subjected to the radiation processing and radiation sterilization.
Thermal oxidation of cross-linked PE and EPR used as insulation materials: Multi-scale correlation over a wide range of temperatures
2021, Polymer Testing
Citation Excerpt :
For both XLPE and XLEPR, the shape of the oxidation products massif is relatively similar but peaking up at different wavenumbers. For XLPE, the maximum at 1720 cm−1 is attributed to ketones [42], and for XLEPR, the maximum at 1714 cm−1 is assigned to carboxylic acids [43]. This difference lies in the fact that the thermo-oxidation mechanisms of the two polymers are slightly different due to the propylene part in XLEPR.
The thermo-oxidation (70–160 °C) of two polymeric materials widely used as electrical insulators in cable systems was studied. The polymers are cross-linked polyethylene (XLPE) and cross-linked ethylene propylene rubber (XLEPR). Characterisation of the aged materials was performed by using FTIR spectroscopy, gel fraction, differential scanning calorimetry and tensile tests. Correlations were observed between the formation of oxidation products, the changes in crystallinity, the variation in gel content and the reduction in mechanical properties in both materials. By using a classical Arrhenius methodology, activation energies of XLPE and XLEPR were calculated in the temperature range 70–160 °C from spectroscopic measurements on 200 μm thick films and from tensile test results on 1 mm thick samples: the average values are 103 kJ mol⁻¹ for XLPE and 102 kJ mol⁻¹ for XLEPR. The results were consistent despite a diffusion-limited oxidation (DLO) effect observed in the 1 mm thick samples.
UV Degradation & Stabilization of Polymers & Rubbers
2020, Handbook of UV Degradation and Stabilization
Stabilization effects of doped inorganic filler on EPDM for space and terrestrial applications
2019, Materials Chemistry and Physics
In this paper a new stabilization method applied to ethylene-propylene-diene monomer (EPDM) against oxidation is presented. The doped lead titanate (PbTiO₃) with several atoms (Ce, Pr, In, Gd and Nb) is a typical example of protection effect of inorganic filler. The isothermal and nonisothermal (CL) procedures were used for monitoring the progresses and the protection degrees of oxidation. The most evident effect was promoted when the filler was doped with gadolinium and dysprosium. The contribution of doped filler as the delay activities in material oxidation is described by the activation energies required for thermal ageing of modified polymer. They were calculated based on temporal kinetic parameters (oxidation induction time, half period of oxidation and maximum oxidation time). The highest stabilization efficiencies were found for the sample containing 5 and 10 phr of inorganic phase. The replacement of organic synthesis antioxidants by doped compounds demonstrates that the choice of inorganic structures may become a usual praxis for the extension of product life time operating in several hazardous circumstances: nuclear engineering, space applications, electrical equipments, where the degradation occurs in a large extent.

View all citing articles on Scopus

View full text

Radiochemical ageing of ethylene–propylene–diene elastomers. 4. Evaluation of some anti-oxidants

Abstract

Introduction

Section snippets

Experimental

Radio-oxidation

Discussion

Conclusion

Polymer Degradation and Stability

Polymer Degradation and Stability

Polymer Photochemistry

Polymer Degradation and Stability

Polymer Degradation and Stability

Nuclear Instruments and Methods in Physical Research B

Nuclear Instruments and Methods in Physical Research B

Polymer Degradation and Stability

J Polymer Science Part A: Polymer Chemistry

Nuclear Instruments and Methods in Physical Research B

Nuclear Instruments and Methods in Physical Research B