Scrap tyre rubber depolymerization by nitrous oxide: products and mechanism of reaction

Abstract

The development of effective methods for reclaiming scrap tyre rubbers attracts a great deal of attention. Earlier, we have proposed an alternative way for reclaiming waste rubber, which consists in its treatment with nitrous oxide (N2O) at 180–230 °C and pressure of 2–5 MPa. In the present study, the reclaimed products obtained by this method from tyre crumb rubber were characterized for the first time by TGA, FTIR, NMR, and ultimate analysis techniques. The results showed that the sol fraction isolated from the reclaimed rubber consists mainly of diene oligomers with the carbonyl, predominantly ketone, groups. The remaining gel fraction includes some of the rubber polymer components and all the carbon black filler present in the initial vulcanizate. Based on the experimental results, a mechanism of vulcanized rubber interaction with nitrous oxide was suggested. The mechanism involves a selective 1,3-dipolar cycloaddition of N2O molecules to C=C bonds of isoprene and butadiene units in vulcanized elastomers. The main reaction route proceeds without cleavage of initial C=C bonds leading to the formation of ketone groups randomly distributed along the polymer backbone. The cleavage of a certain amount of reacting C=C bonds leads to controllable scission of macromolecular chains and accordingly to depolymerization of the initial vulcanized rubber, thus yielding a plastic reclaim product. The sulfur cross-links do not appear to be broken in the course of waste rubber treatment.