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The Preparation and Properties of Thermo-reversibly Cross-linked Rubber Via Diels-Alder Chemistry

Published: August 25th, 2016



1Department of Chemical Engineering, University of Groningen, 2Dutch Polymer Institute (DPI)

A simple two-step approach involving rubber modification and cross-linking yields fully reworkable, elastic rubber products.

A method for using Diels Alder thermo-reversible chemistry as cross-linking tool for rubber products is demonstrated. In this work, a commercial ethylene-propylene rubber, grafted with maleic anhydride, is thermo-reversibly cross-linked in two steps. The pending anhydride moieties are first modified with furfurylamine to graft furan groups to the rubber backbone. These pendant furan groups are then cross-linked with a bis-maleimide via a Diels-Alder coupling reaction. Both reactions can be performed under a broad range of experimental conditions and can easily be applied on a large scale. The material properties of the resulting Diels-Alder cross-linked rubbers are similar to a peroxide-cured ethylene/propylene/diene rubber (EPDM) reference. The cross-links break at elevated temperatures (> 150 °C) via the retro-Diels-Alder reaction and can be reformed by thermal annealing at lower temperatures (50-70 °C). Reversibility of the system was proven with infrared spectroscopy, solubility tests and mechanical properties. Recyclability of the material was also shown in a practical way, i.e., by cutting a cross-linked sample into small parts and compression molding them into new samples displaying comparable mechanical properties, which is not possible for conventionally cross-linked rubbers.

Sulfur vulcanization and peroxide curing are currently the main industrial cross-linking techniques in the rubber industry, yielding irreversible chemical cross-links that prevent melt reprocessing.1, 2 A 'cradle to cradle' approach to recycle cross-linked rubbers requires a material that behaves similar to permanently cross-linked rubbers at service conditions, while having the processability and complete recyclability of a thermoplastic at high temperatures. An approach to achieve such recyclability uses rubbery networks with reversible cross-links that respond to an external stimulus, such as temperature (most feasible from the viewpoint of futur....

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1. Rubber Modification

  1. Prepare the maleated EPM (EPM-g-MA, 49 wt% ethylene, 2.1 wt% MA, Mn = 50 kg/mol, PDI = 2.0) rubber and furfurylamine (FFA) before starting the experiment as indicated in steps 1.1.1-
    1. Dry the EPM-g-MA rubber in a vacuum oven for one hour at 175 °C to convert present di-acid into anhydride.11
    2. Compression mold a 0.1 mm thick rubber film in a hot press for 10 min at 150 °C and 100 bar.
    3. Record a transmission infrared sp.......

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The successful modification of EPM-g-MA into EPM-g-furan and the cross-linking with the bismaleimide is shown by Fourier transform infrared spectrometry (FTIR) (Figure 2). The presence of furan groups in the EPM-g-furan product can be deduced from the splitting of the CC aliphatic stretching peak (Equation = 1,050 cm-1) into two furan peaks (Equation

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A commercial EPM-g-MA rubber was thermo-reversibly cross-linked in a simple two-step approach. The maleated rubber was first modified with FFA to graft furan groups onto the rubber backbone. The resulting pending furans show reactivity as Diels-Alder dienes. An aliphatic BM was used as cross-linking agent, resulting in a thermo-reversible bridge between two furan moieties. Both reactions were successful with good conversions (> 80%) according to infrared spectroscopy, elemental analysis. Cross-linking was shown by sw.......

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This research forms part of the research program of the Dutch Polymer Institute, project #749.


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Name Company Catalog Number Comments
ENB-EPDM LANXESS Elastomers B.V. Keltan 8550C
EPM-g-MA LANXESS Elastomers B.V. Keltan DE5005 Vacuum oven for one hour at 175 °C 
furfurylamine Sigma-Aldrich F20009 Freshly distillated before use
di-dodecylamine Sigma-Aldrich 36784
maleic anhydride Sigma-Aldrich M0357
octadecyl-1-(3,5-di-tert.-butyl-4-hydroxyphenyl)propionate Sigma-Aldrich 367079
bis(tert.-butylperoxy-iso-propyl) benzene Sigma-Aldrich 531685
tetrahydrofuran Sigma-Aldrich 401757
decalin Sigma-Aldrich 294772
acetone Sigma-Aldrich 320110

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