The pyrolysis of End-of-Life Tyre (ELT) to produce recycled carbon black (r-CB) is well accepted globally as one of the efficient means to combat pollution and improve waste management related to the environmental carbon footprint. Currently, the high ash content inherent in the r-CB is a major concern for rubber compounders. The high ash content in r-CB is mainly composed of carbonaceous materials, silicone dioxide and zinc oxide that might interfere with the sulphur-vulcanisation reaction. The first step in addressing this concern was to evaluate the influence of high ash on the cure characteristics of Natural Rubber filled with r-CB. These characteristics would throw some light on the extent of crosslinking that is affecting the physical properties of the vulcanised rubber. Two natural rubber compounds were prepared, one was filled with r-CB having high ash, and the second was filled with HAF black having very low ash. The loading was fixed at 50 phr. The cure characteristics test using the oscillating disc rheometer was done at 150 °C. The results showed that the r-CB-filled NR compound gave lower minimum torque (Tmin) and lower maximum torque (Tmax) than that of the HAF-filled NR compound. The difference in torque ΔT of the r-CB-filled compound was only 52% that of the ΔT of the HAF-filled compound. The r-CB has a higher ION (129 g/kg) than the ION of HAF (82 ± 5 g/kg), indicating that the former has a smaller particle size than that of the latter. One would expect the r-CB to give higher rubber compound viscosity and hence higher Tmin, Tmax and ΔT than that of the HAF rubber compound. The results show the contrary, indicating that the influence of ash content on sulphur vulcanisation outweighed the physical effects of particle size and black structure. The low ΔT of r-CB is envisaged to be attributed to low crosslink concentration. An equilibrium swelling test was carried out to determine the crosslink concentration. The results showed that the crosslink concentration of r-CB vulcanised rubber was lower by 60% than that of HAF vulcanised rubber. Having low crosslink concentration, the tensile strength of r-CB-filled NR was 13.6 MPa in contrast, the tensile strength of HAF-filled NR was 21.1 MPa. The second step of the investigation was to increase the crosslink concentration of r-CB filled NR compound through the incorporation of ethylene glycol (EG), coupling agent Bis[3-triethoxysilylpropyl-]tetrasulfide (Si69) and extra dosage of sulphur to compensate for the loss of sulphur trapped through the pores of the silicas, and to enhance the rubber-filler interactions between the silica particles and the rubber matrix. When these necessary actions were taken, the crosslink concentration of r-CB-filled compound was increased from 1.86 × 10− 2 mol/kg to 5.52 × 10− 2 mol/kg and concomitantly the tensile strength of r-CB filled vulcanisate increased to 21.2 MPa, comparable to that of HAF vulcanisate, 21.1 MPa.

The effects of high ash content of recycled carbon black on the cure characteristics and on the physical properties of vulcanised natural rubber

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发表日期：

2026年6月19日

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Azemi B. Samsuri

摘要

The pyrolysis of End-of-Life Tyre (ELT) to produce recycled carbon black (r-CB) is well accepted globally as one of the efficient means to combat pollution and improve waste management related to the environmental carbon footprint. Currently, the high ash content inherent in the r-CB is a major concern for rubber compounders. The high ash content in r-CB is mainly composed of carbonaceous materials, silicone dioxide and zinc oxide that might interfere with the sulphur-vulcanisation reaction. The first step in addressing this concern was to evaluate the influence of high ash on the cure characteristics of Natural Rubber filled with r-CB. These characteristics would throw some light on the extent of crosslinking that is affecting the physical properties of the vulcanised rubber. Two natural rubber compounds were prepared, one was filled with r-CB having high ash, and the second was filled with HAF black having very low ash. The loading was fixed at 50 phr. The cure characteristics test using the oscillating disc rheometer was done at 150 °C. The results showed that the r-CB-filled NR compound gave lower minimum torque (T_min) and lower maximum torque (T_max) than that of the HAF-filled NR compound. The difference in torque ΔT of the r-CB-filled compound was only 52% that of the ΔT of the HAF-filled compound. The r-CB has a higher ION (129 g/kg) than the ION of HAF (82 ± 5 g/kg), indicating that the former has a smaller particle size than that of the latter. One would expect the r-CB to give higher rubber compound viscosity and hence higher T_min, T_max and ΔT than that of the HAF rubber compound. The results show the contrary, indicating that the influence of ash content on sulphur vulcanisation outweighed the physical effects of particle size and black structure. The low ΔT of r-CB is envisaged to be attributed to low crosslink concentration. An equilibrium swelling test was carried out to determine the crosslink concentration. The results showed that the crosslink concentration of r-CB vulcanised rubber was lower by 60% than that of HAF vulcanised rubber. Having low crosslink concentration, the tensile strength of r-CB-filled NR was 13.6 MPa in contrast, the tensile strength of HAF-filled NR was 21.1 MPa. The second step of the investigation was to increase the crosslink concentration of r-CB filled NR compound through the incorporation of ethylene glycol (EG), coupling agent Bis[3-triethoxysilylpropyl-]tetrasulfide (Si69) and extra dosage of sulphur to compensate for the loss of sulphur trapped through the pores of the silicas, and to enhance the rubber-filler interactions between the silica particles and the rubber matrix. When these necessary actions were taken, the crosslink concentration of r-CB-filled compound was increased from 1.86 × 10^− 2 mol/kg to 5.52 × 10^− 2 mol/kg and concomitantly the tensile strength of r-CB filled vulcanisate increased to 21.2 MPa, comparable to that of HAF vulcanisate, 21.1 MPa.