Thermo-reversible crosslinked natural rubber: A Diels-Alder route for reuse and self-healing properties in elastomers

Natural rubber (NR) is one of Nature's most exceptional materials and is, therefore, widely used by industry, in particular by the tire industry. In spite of the interest in recycling vulcanized NR, there is no suitable way to do it and most of the tires are discarded in landfills. Here, we examine the use of a Diels-Alder reaction to obtain a NR fully and reversibly crosslinked and with self-repair capacity. NR was crosslinked via Diels-Alder (DA) reactions following a three-step procedure: i) NR was reacted with maleic anhydride; ii) furan moieties were then grafted to the maleated rubber; iii) pending furans were further crosslinked with a bismaleimide, resulting in a thermoreversible bridge which led to a crosslinked NR. The formation of the DA adduct was confirmed and the reversibility of the DA reaction was demonstrated through dynamic-mechanical analysis and rheological measurements. The self-healing capacity of the material was also evaluated; qualitatively, at different scales by scanning electron microscopy, optical profilometry and atomic force microscopy and; quantitatively, by tensile testing obtaining 80% of strength recovery at small deformations. We provide phenomenological explanations of the reversibility and healing performance of the DA reaction too. This research provides a framework to crosslink rubbers, yielding a straightforward strategy for both reusing and extending the lifetime of elastomeric goods.