Chemical Modifications in Styrene–Butadiene Rubber after Microwave Devulcanization

Abstract

Microwave devulcanization has been studied as a method for elastomer recycling, which is based on the conversion of the reticulated and infusible structure of thermosetting rubbers in free polymeric chains able to be remolded by thermomechanical processing in recycling operations for the manufacture of other products. Elastomeric wastes are often irregularly discarded in nature, producing serious environmental damage, and their mechanical recycling is still considered a challenge. Thus, the development of alternatives for elastomer recycling is directly related to the actions of sustainable development and economic benefits to companies that pay to discard their wastes. The aim of this work is to evaluate the chemical modifications occurring in styrene–butadiene rubber (SBR) after microwave devulcanization. Compounds of SBR were vulcanized in the presence of vulcanization agents and variable amounts of carbon black, and then the rubbers were milled and submitted to microwave treatment. Only the SBR with high carbon black content shows some portion of devulcanized material. However, the rubber with lower content of carbon black which was devulcanized by microwave radiation shows an increase in cross-link density. The microwave treatment also causes cross-link breaks mainly in polysulfidic bonds as well as decomposition of chemical groups containing sulfur attached to the chemical structure of SBR, while the chemical bonds of higher energy such as monosulfidic bonds remain preserved. The improvement of the microwave method for rubber devulcanization represents a way for viable recycling of thermosetting rubbers.

Devulcanization of ground tire rubber: Physical and chemical changes after different microwave exposure times

“…It is also interesting to notice that the bands at 1076, 1029 and 1006 cm -1 , which are related to C-S bonds, are seen mainly in the GTRv and GTR3 samples. Hirayama and Saron [25] also noticed changes in the intensity bands in the SBR samples most exposed to microwaves. The authors observed that polysulfidic bonds were broken, however the monosulfidic bonds remained preserved.…”

“…According to spectra (Figure 2), the intensities of some bands were altered with the devulcanization process. Previous studies have attributed these IR bands to: (1) 1537 cm -1 : stretching vibration of a methyl-assisted conjugated double bond (" -(C=C) n -, where n > 6) from both NR [30,31] and SBR [32]; (2) deformation from NR rubber [31]; (3) 1076 cm -1 : symmetric C-S-C group stretching vibrations in the two C-S bonds [33]; (4) 1029 and 1006 cm -1 : C-S bonds [25]; (5) 962 cm -1 : -CH=CH-group vibrations of butadiene present in SBR [33,34]; and (6) 828 cm -1 : $ C-H bending from NR rubber [31]. The results reveal that the samples most exposed to microwaves had lower intensity bands, which was more evident in the GTR7 sample.…”

“…The results showed that this mixture can be used in automotive profiles obtained by compression molding. Other papers analyzed the influence of the amount of carbon black in the rubber during the microwave devulcanization and concluded that the degree of devulcanization increases as a function of the increase in the amount of carbon black [23,25]. Paulo et al [26] verified that the microwave devulcanization, using inorganic salts and nitric acid, promoted the breakup of sulfur cross-links and the oxidation in the SBR.…”

Blends of ground tire rubber devulcanized by microwaves/HDPE - Part A: influence of devulcanization process

“…As the ML value is proportional to the initial viscosity of the sample [19] , the increase of the exposure time to microwaves reduced the viscosity of rubber induced by the breaking of the three-dimensional network of the vulcanized GTR. The sample GTR3 presented the highest ML value probably due to the formation of new bonds in the rubber, since during sample exposure to microwaves, bonds can be broken and created at the same time [20] . Therefore, depending on the time at which the sample was exposed to microwaves, the former event can exceed the later one or vice-versa.…”

The Role of Carbon Black on Devulcanization of Natural Rubber by Microwaves

“…Devulcanization is a way of providing energy to destroy, total or partially, the three-dimensional network formed during vulcanization 4 . The known methods that use different ways to promote the devulcanization of rubber include thermomechanical, thermo-chemical and mechano-chemical [5][6][7][8][9] , physical (by microwaves 3,[10][11][12][13][14][15][16][17][18][19][20] and ultrasound [21][22][23][24][25][26][27][28][29] ) and biochemical [30][31][32][33] . All of them involve complex transformations that lead to depolymerization, oxidation, and in many cases, the degradation of polymeric chains of rubber, with the consequent reduction on its viscosity 34 .…”