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

Summary

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

Abstract

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.

Introduction

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 future indust....

Protocol

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 1.1.1-1.1.4.⁵
   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 spectrum of the resulting film after placing it in a KBr tablet holder.
      NOTE: The conversion of the hydrolyzed di-acid into anhydride is co....

Results

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 ( = 1,050 .......

Discussion

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.......

Materials

List of materials used in this article

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