In a recent study published in the journal PolymersChinese researchers have demonstrated a new method for recycling waste produced by the rubber industry.
Study: Selective decomposition of rubber waste from the footwear industry by the combination of a thermal process and mechanical grinding. Image Credit: cornerstock/Shutterstock.com
Rubber industry and waste
The rubber sector contributes significantly to the global economy and the production of rubber from the footwear and tire industries is increasing every year. The significant amount of waste produced during the rubber manufacturing process is one of the critical environmental issues facing the rubber industry.
For this reason, the disposal of rubber waste remains an economic, social and ecological challenge worldwide. Improper disposal of waste rubber leads to the combustion of a considerable volume of waste, which causes air pollution. Additionally, waste rubber also increases the risk of disease transmission and reduces durability.
In the present study, waste rubber (WR) from the footwear industry was crushed and recovered using a torque rheometer, and the effects of various fill rates and crushing temperatures on the structure of WR were analyzed.
Torque (M) of WR samples as a function of grinding time at different temperatures. Image Credit: Xiao, Q et al., Polymers
The materials used for the study were WR, xylene and tetrahydrofuran (THF), analytical reagents and styrene-butadiene rubber. The main ingredients in rubber were carbon black (CB), stearic acid, zinc oxide, and sulfur.
Initially, the WR was washed and dried before being ground into particles in a grinder to prepare the samples. Additionally, WR was ground in a torque rheometer with two roller rotors at different temperatures and fill rates. During the grinding process, the rotor speed was set to 40 rpm and the grinding time to 10 minutes.
In addition, the rheological torque of the samples was continuously measured at varying temperatures and filling rates. Overall, reclaimed rubber scrap (RWR) and styrene-butadiene rubber (SBR) were mixed in different formulations to produce six samples.
Fourier Transform Infrared Spectroscopy (FTIR), Laser Particle Size Analyzer, Soil Fraction Analysis, Gel Permeation Chromatography (GPC), Differential Scanning Calorimeter (DSC), and Electron Microscope scanning (SEM) have been used by researchers to study the evolution of the structure, particle size distribution and morphology of recycled rubber.
SEM images of the fractured surface at different WR temperatures: (a) untreated; (b) at 30°C; (vs) at 80°C; and (D) at 140°C. Image Credit: Xiao, Q et al., Polymers
The results demonstrated that temperature and filling rate have a significant impact on the chemical and physical structure of WR. As the grinding temperature and sol fraction increased, the sample energy consumption and glass transition temperature decreased. In addition, the soil fraction and energy consumption of the sample increased as the particle size decreased.
The thermomechanical approach used by the researchers reduced the particle size of WR, which is an essential parameter for recycled rubber. The study of the mechanism carried out by the researchers demonstrated that the major break in the chain was erased by the grinding temperature and the filling rate. It should be noted that the reason for the severe chain break was the oxidation reaction that occurs during the thermomechanical grinding process. Moreover, according to the researchers, Rosin’s equation could describe the particle size distribution of WR after it has been ground.
The results further indicated that the optimum impact of thermomechanical grinding was achieved at a temperature of 80 °C and a filling rate of 85%. The soil fraction, DSC and GPC results demonstrated that the heat produced during the mechanical grinding caused the reticulation fracture and the primary chain fracture.
The revulcanization characteristics of the samples showed a similar trend to that of pure SBR. The increase in MH depended on the RWR content and the SBR, RWR with degree of recovery exhibited high dispersibility and reinforcement. Moreover, the addition of RWR to SBR did not show any negative effect on the mechanical qualities but improved in the case of SBR.
The SEM results demonstrated that the untreated WR had a smooth surface and that grinding produced uneven particles with a rough surface. Compared to WR particles ground at 80 °C and 140 °C, the morphology of the WR particles ground at 30 °C showed smaller fractures on the surface. The rubber particles become coarser as the grinding temperature increases. Overall, the SEM results showed that the crosslinked rubber was under high pressure and shear stress during the grinding process. Moreover, the network inhomogeneity caused the stress concentration which led to the failure of the network chains in the samples.
SEM images of RWR/SBR composites: (a) 100SBR/0RWR; (b)90SBR/10RWR; (vs)80SBR/20RWR; (D)70SBR/30RWR; and (and) 60SBR/40RWR. Image Credit: Xiao, Q et al., Polymers
To summarize, the thermomechanical method reduced the particle size of WR, and Rosin’s equation can be used to characterize the particle size of WR after it has been ground. Oxidation reaction, thermomechanical grinding process, sol proportion, grinding temperature and filling rate are critical parameters to produce high quality recycled rubber.
Additionally, recycled rubber produced with SBR exhibited vulcanization properties similar to pure SBR. The technique of selective decomposition of waste rubber demonstrated by the researchers in the present study can be used to reuse the WR produced in the footwear industry.
Xiao, Q.; Cao, C.; Xiao, L.; Bail.; Cheng, H.; Lei, D.; Sun, X.; Zeng, L.; Huang, B.; Qian, Q.; et al. Selective decomposition of rubber waste from the footwear industry by the combination of a thermal process and mechanical grinding. Polymers 2022, 14, 1057. https://www.mdpi.com/2073-4360/14/5/1057