Devulcanization of Nitrile Butadiene Rubber in Nitrobenzene Kohji Masaki, Shin-Ichi Ohkawara, Tomohiro Hirano, Makiko Seno, Tsuneyuki Sato Department of Chemical Science and Technology, Faculty of Engineering, Tokushima University, Minamijosanjima 2-1, Tokushima 770-8506, Japan Received 10 February 2003; accepted 19 August 2003 ABSTRACT: Sulfur-crosslinked nitrile butadiene rubber (s- NBR) was found to be devulcanized when it was heated with nitrobenzene at 200°C for 3 h. The tetrahydrofuran (THF)- soluble fraction from s-NBR heated with nitrobenzene was purified by reprecipitation with THF/n-hexane, chloroform/ n-hexane, and THF/n-hexane systems and was then character- ized by means of Fourier transform infrared (FTIR) spectros- copy, 1 H-NMR, gel permeation chromatography, dynamic thermogravimetry/differential thermal analysis (DTA), and differential scanning calorimetry (DSC). FTIR and 1 H-NMR results revealed that the THF-soluble fraction contained aro- matic rings derived from nitrobenzene. Furthermore, the mo- lecular weight of the THF-soluble fraction was much lower than that of the parent noncrosslinked poly(acrylonitrile-co- butadiene). Although the weight loss of THF-soluble fraction began at a lower temperature than that of the nonheated orig- inal nitrile butadiene rubber, the residual weight at 700°C tended to be higher for the former. This tendency became more marked with increasing time of heat treatment with nitroben- zene. The DSC-determined glass-transition temperature of the THF-soluble fraction was higher than that of the original s- NBR. To elucidate the devulcanization mechanism, we inves- tigated two types of model reactions; one was the reaction of diphenyl disulfide with nitrobenzene, and the other was the reaction of polybutadiene with nitrobenzene. The former reac- tion, carried out at 250°C in diphenyl ether, yielded diphenyl sulfide with a loss of diphenyl disulfide and nitrobenzene. The use of a higher molar ratio of nitrobenzene to diphenyl disul- fide resulted in a depression of diphenyl sulfide formation. The reaction of p-chloronitrobenzene with diphenyl disulfide also gave diphenyl sulfide. The reaction of polybutadiene with nitrobenzene at 200°C resulted in the backbone scission of the polymer. The THF-soluble solid product of the latter model reaction was found by FTIR and 1 H-NMR to contain aromatic rings derived from nitrobenzene. The devulcanization mecha- nism is discussed on the basis of a comparison of the results of the model reactions with those of the s-NBR devulcanization. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3342–3353, 2004 Key words: polybutadiene; devulcanization; nitrobenzene; rubber; networks INTRODUCTION Vulcanized rubbers with three-dimensional chemical networks are infusible and insoluble in organic sol- vents. The presence of these networks causes a serious problem in rubber recycling. There are two main ap- proaches to rubber recycling. One approach is me- chanical methods (e.g., thermomechanical, cryome- chanical). Recently, ultrasonic technology 1–7 was re- ported to be effective for waste vulcanized rubbers as a mechanical processing method. The other approach is chemical processing, 8 which is a possible method for reversing the crosslinked network through the use of chemical agents 9 that attack carbon–sulfur or sul- fur–sulfur bonds. In the rubber industry, nitrile butadiene rubber (NBR) is widely used because of its oil-resistance char- acteristics (e.g., O rings, packings, rotary shaft lip seals). NBR is a crosslinked rubber of poly(acryloni- trile-co-butadiene) (AB copolymer). Recently, we found that sulfur-crosslinked nitrile butadiene rubber (s-NBR) was easily devulcanized when it was heated with nitrobenzene at 200°C for several hours to give tetrahydrofuran (THF)-soluble solid products. To our knowledge, there have been no reports on the devul- canization of s-NBR by such an approach. This led us to investigate the devulcanization mechanism of s- NBR by heating with nitrobenzene. This article describes the results of the devulcaniza- tion of s-NBR by heating with nitrobenzene. To eluci- date the devulcanization mechanism, two types of the model reactions were investigated. One was the reac- tion of diphenyl disulfide with nitrobenzene at 250°C in diphenyl ether, and the other was the reaction of polybutadiene with nitrobenzene at 200°C. EXPERIMENTAL Mesurements Fourier transform infrared (FTIR) spectra of the reac- tion products were obtained with a Jeol JIR-6500 spec- Correspondence to: T. Sato (sato@chem.tokushima-u.ac.jp). Contract grant sponsor: Satellite Venture Business Labo- ratories, Tokushima University. Journal of Applied Polymer Science, Vol. 91, 3342–3353 (2004) © 2004 Wiley Periodicals, Inc.