Elsevier

Radiation Physics and Chemistry

Volume 94, January 2014, Pages 22-25
Radiation Physics and Chemistry

Radiation-induced degradation of an epoxy thermoset supported by hydrogen peroxide

https://doi.org/10.1016/j.radphyschem.2013.04.014Get rights and content

Highlights

  • Mechanism of the radiation induced degradation of epoxy resin was proposed.

  • Epoxy thermoset swelled with H2O2 is radiation susceptible.

  • Radiation treatment followed by H2O2 swelling can be applied for resin recycling.

Abstract

Epoxy resin was decomposed applying two complementary treatments, namely (1) soaking up with 30% aqueous solution of hydrogen peroxide and (2) exposure of the swelled material to ionizing radiation in air atmosphere over doses up to 1000 kGy. The phase transition characteristic determined by the DSC technique revealed that both factors, swelling with oxidizing agent and irradiation applied sequentially induce in the resin deep structural changes resulting in the rise of two new exothermic transitions assigned to hydrogen peroxide decomposition and resin oxidation. The flexural stress at break measurements confirmed significant influence of H2O2 on the mechanical properties of the irradiated material which, under applied conditions, is efficiently decayed via oxidative degradation. On the basis of results obtained by EPR spectroscopy chemical mechanism of the radiation induced degradation was proposed.

Introduction

Epoxy resins are widely used in many branches of industry (Alessis et al., 2007, Benfarhi et al., 2004, Dispenza et al., 2002). Therefore, development of the effective technologies for their recycling is necessary, especially that thermosets can be recycled neither by remelting in the molding process nor by depolymerization to their original constituents. One of the methods under consideration is a radiation treatment (Burillo et al., 2002, Czvikovszky, 1995, Davenas et al., 2002) that in some cases might facilitate degradation and further reprocessing some polymeric materials, e.g. butyl rubber. However, epoxy resins demonstrate unusual radiation resistance resulting not only from their chemical structure but also from relatively low oxygen diffusion coefficient that reduces oxidative degradation of the material. Longiéras et al. (2007) studied a distribution of the oxidation products across irradiated epoxy resin films. They found that upon irradiation with a dose of 5 MGy only 20 μm external layer experienced oxidation leading to the formation of carboxylic terminal groups and that below a few MGy decomposition of bulk epoxy resin was negligible. Consequently, radiation technology cannot be directly implemented due to extremely high energy consumption and costs of the operation, and the process needs appropriate pretreatment.

Reported investigations were focused on the decomposition of diglycidyl ether of bisferol-A (DGEBA) hardened with triethylene tetramine (TETA). Upon swelling with hydrogen peroxide solution the resins were irradiated with electron beam in air atmosphere. The degradation extent was characterized by the comparison of the results obtained for neat epoxy resin and irradiated one, swelled previously with hydrogen peroxide aqueous solution. We expected that incorporation of the readily available and inexpensive liquids into epoxy resins enhanced the range of oxidative degradation initiated by ionizing radiation providing an attractive solution to the problem of the thermoset recycling.

Section snippets

Materials

Epoxy monomer (diglycidyl ether of bispherol-A, DGEBA) and hardener (triethylene tetraamine, TETA), both purchased from Aldrich Co, were compounded in a ratio of 10:1. The prepared mixture was poured into the aluminum molds in the form of bars and submitted to a two-step temperature program: the sam-ples were kept at 25 °C for 16 h, and subsequently at 95 °C for 2 h. Dimensions of the tested samples were as follows: 4×10×120 mm3. The process allowed to achieve fully cured and thermally stable epoxy

Swelling

The relationships between swelling of the epoxy resin samples kept either in water or in 30% aqueous solution of H2O2 and the time of swelling are shown in Fig. 1. Water uptake did not exceed 1% and seemed to be insufficient from practical point of view. Swelling of the epoxy resin with H2O2aq after 33 days achieves about 6% but diffusion of the molecules has to be relatively slow as the correlation shows that after that time saturation has not been reached yet. Relatively high sorption effect

Conclusions

As was found by DSC measurements, the epoxy resin during swelling with 30% H2O2aq, is penetrated predominantly by hydrogen peroxide molecules whereas diffusion of H2O is negligible.

After H2O2 sorption, bending strength of the epoxy resin is reduced only by 10% thus the degradation effect is limited. Nonetheless, if the swelled material is exposed to ionizing radiation its strength diminishes radically and mechanical properties deteriorate considerably. These effects result from oxidation of the

Acknowledgment

This work was supported by a project No PBZ-MNiSW-5/3/2006 from the Polish Ministry of Science and Higher Education.

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