Ultrasonic devulcanization of unfilled SBR under static and continuous conditions

A static ultrasonic treatment device was used to investigate the effect of ultrasound on devulcanization of unfilled SBR disks in the absence of shearing effect. Some visible bubbles were observed at the center of the disk sample at low pressure, as well as at the edges at high pressure, as the treatment time increases. Evidently, these bubbles are a proof of the existence of cavitation during the ultrasonic devulcanization. Stronger reduction of crosslink density observed at a higher pressure in the treatment is a direct indication of the significant role of pressure in improving the efficiency of ultrasonic devulcanization. Treatment of the ground sample, having higher concentration of voids, indicated lower crosslink density and gel fraction than those of the sheet sample not having visible voids. The unique correlation between gel fraction and crosslink density of devulcanized rubbers observed earlier in continuous process, was also obtained in the present case. However, this curve is shifted towards higher values of crosslink density. It is concluded that shearing effect has an influence on improving the efficiency of devulcanization.