Neste trabalho é descrita a desvulcanização da borracha SBR (Styrene Butadiene Rubber) via oxidação controlada com ácido nítrico. Análises por IV, RMN de 1 H e 13 C, CHN, TG, GPC e experimentos de DTP-EM (Decomposição a Temperatura Programada -Espectrometria de Massas) sugerem que o ácido nítrico promove dois processos: (i) a desvulcanização por oxidação das pontes de enxofre e (ii) a oxidação da cadeia carbônica levando à formação de grupos -COOH e -NO 2 . Em condições mais drásticas de reação, a cadeia polimérica é fortemente oxidada e se fragmenta formando uma fração altamente oxigenada, de baixo peso molecular e solúvel em água.This work describes the devulcanization of vulcanized SBR (Styrene Butadiene Rubber) based on controlled oxidation with nitric acid. IR, 1 H and 13 C-NMR, CHN elemental analyses, TG, GPC and TPD-MS (Temperature Programmed Decomposition-Mass Spectrometry) experiments suggest that nitric acid promotes two processes: (i) devulcanization by oxidation of the sulfide crosslinks and (ii) oxidation of the carbon framework leading to the formation of -COOH and -NO 2 groups. Under aggressive conditions (HNO 3, reflux) the polymer chain is strongly oxidized and fragmented to a highly oxygenated low molecular weight fraction, which is soluble in water.
Keywords: devulcanization, HNO 3 , oxidation
IntroductionThe disposal and reuse of waste tires remain not only a serious environmental concern but also an economical issue. Several alternatives, such as direct uses of scrap tires, combustion to produce energy, pyrolysis to produce fuels, the use of crumbles to asphalt roads and devulcanization to recycle the polymer have been extensively investigated in the last decades. Different chemical, physical and biological processes for the devulcanization of vulcanized rubber have been reported in the literature. 1 For the chemical devulcanization processes an extensive variety of agents such as: sulphides, e.g. diphenyl-disulphides, mercaptans, thiophenol, phosphorus compounds, e.g. triphenylphosphine and dibutylphosphite have been used. 2-13 Also, some reducing compounds have been studied for the devulcanization process, such as lithium aluminum hydride, 14 phenyl lithium, 15 CuCl/Bu 3 N, 16 alkaline metals (e.g. Na o ) and FeCl 2 / hydrazine. 17 On the other hand, oxidative processes for the devulcanization have been scarcely investigated. One example is the use of CHCl 3 /O 3 /H 2 O 2 , which showed low efficiency and high cost. 18 Nitric acid, HNO 3 , is a strong oxidizing agent which is well known to react with all types of sulfur groups, 19 offering the possibility to devulcanize rubber by oxidizing the sulfide crosslinks. The corrosion effect of nitric acid on rubber surface has been observed in 1985 by scanning electron microscopy. 20 Also HNO 3 and 30% H 2 O 2 have been used for the modification of the surface properties of rubber powder used as filler. 21 However, the use of the controlled oxidation with HNO 3 to devulcanize vulcanized rubbers has not yet been studied.In this work a detailed inv...
