Decrosslinking of Crosslinked High Density Polyethylene Via Ultrasonic Assisted Extrusion

Among the many environmental problems which mankind faces in the XXI century is the problem of environmental sustainability and management of the tremendous amount of generated polymer waste. Among various polymer wastes, management of crosslinked plastics is a major environmental problem requiring a solution. This study is specifically directed toward the creation of a new, environmentally friendly process for decrosslinking of crosslinked HDPE via ultrasonic assisted extrusion. Extrusion of crosslinked and virgin HDPE at various processing conditions are performed by means of ultrasonic single and twin screw extruders. Die pressure increases with the flow rate and decreases with the amplitude. Ultrasonic power consumption increases with both flow rate and amplitude. Without the imposition of ultrasound the high flow rate in the single screw extruder was impossible to achieve due to an excessive torque. Gel fraction, crosslink density, dynamic properties, thermal and mechanical properties of decrosslinked and virgin HDPE is measured. Significant drops of gel fraction and crosslink density of decrosslinked HDPE was observed indicating the occurrence of  decrosslinking. An universal linear relation between the normalized gel fraction and the normalized crosslink density is found, regardless of the type of extruders and processing conditions. The decrosslinking effect induced by ultrasound increased with increase of the ultrasonic amplitude which was higher in the twin screw extruder. The  increase of amplitude led to a decrease of the complex viscosity and storage modulus and a increase of loss tangent of decrosslinked HDPE. The molecular structure of treated virgin HDPE was not affected by ultrasonic treatment which was consistent with the observation on dynamic characteristics. However, dynamic characteristics of sol indicated an increase in polymer chain branching due to ultrasonic decrosslinking effect, as detected by an increase of the activation energy of viscous flow, which was higher in the samples obtained from the single screw extruder. Also, a higher viscosity of sol was observed in HDPE decrosslinked by the twin screw extruder. The viscosity-frequency curves were well described by a power-law model with the consistency and power law indices linearly increasing and decreasing with crosslink density, respectively. The mechanical properties of decrosslinked HDPE showed a strong dependence on the type of extruder and a little dependence on processing conditions and molecular characteristics.