Decrosslinking of crosslinked high‐density polyethylene via ultrasonically aided single‐screw extrusion

Abstract

A novel environmentally friendly process for decrosslinking of the peroxide crosslinked HDPE (XHDPE) via ultrasonic assisted single (SSE) is developed and its process characteristics are established. The specific ultrasonic energy decreases with the flow rate and increases with the ultrasonic amplitude, while die pressure increases with the flow rate and decreases with the ultrasonic amplitude. Application of ultrasonic treatment during extrusion enables an increase of productivity. Gel fraction, crosslink density, dynamic and mechanical properties, and thermal behavior of the virgin HDPE, XHDPE, and decrosslinked XHDPE are measured. Gel fraction and crosslink density of the decrosslinked XHDPE are decreased with increasing flow rate and ultrasonic amplitude. A unique linear relation between the normalized gel fraction and the normalized crosslink density is found, regardless of the type of extruders and processing conditions. SEM images reveals that the decrosslinked XHDPE is a composite of submicron size gel particles embedded in its sol matrix. The sol extracted from the decrosslinked XHDPE exhibits a higher complex viscosity and higher level of branching than the virgin HDPE. An increase of the ultrasonic amplitude leads to a decrease of the complex viscosity, storage and loss moduli, and an increase of the loss tangent of the decrosslinked XHDPE. The thermal behavior and mechanical properties of the decrosslinked XHDPE show a weak dependency on processing conditions. At some processing conditions, mechanical properties of the decrosslinked XHDPE are close or higher to those of XHDPE.

Comparison between decrosslinking of crosslinked high and low density polyethylenes via ultrasonically aided extrusion

“…It is seen from Figure 4 that the barrel pressure and die pressure decrease with the ultrasonic amplitude. The decrease of barrel pressure in SSE with the amplitude is due to the decrease of viscosity caused by thixotropic effect and permanent decrease of the gel fraction and crosslink density introduced by ultrasonic trreatment [9]. The decrease of die pressure in TSE with the amplitude is due to the permanent decrease of viscosity caused by reduction of the gel fraction and crosslink density [11].…”

“…Since the value of n M is unavailable, the weight average molecular weight, w M =5.2 10 4 g/mol [9], is used in Figure 4. Normalized gel fraction versus the normalized crosslink density of decrosslinked XHDPE and XLDPE.…”

“…The ultrasonically aided extrusion of XHDPE is studied including the effects of flow rate [8,9], extruder type [10], screw design [11] and degree of crosslinking [12]. It is found that the ultrasonically aided extrusion is a promising technology to obtain a decrosslinked XHDPE exhibiting good processibility and superior mechanical performance.…”

“…The SSE was equipped with two water-cooled ultrasonic horns operating at an ultrasonic frequency of 20 kHz connected to boosters and converters [9]. A 16 mm ultrasonic TSE equipped with a circular die of a diameter of 3 mm and a length of 11 mm was used.…”

Comparison between decrosslinking of crosslinked high and low density polyethylenes via ultrasonically aided extrusion

“…However, a severe thermal degradation occurred due to significant ultrasonic energy dissipation in the ultrasonic treatment zone leading to inferior tensile properties of decrosslinked XHDPE. To overcome this problem, two novel ultrasonically aided extrusion systems with better controlled cooling of ultrasonic treatment zone are developed [13] and used to perform ultrasonic decrosslinking of XHDPE [14][15][16][17]. The effects of ultrasonic amplitude [14][15][16][17], flow rate [14,15], extruder type [15], screw design [16] and degree of crosslinking [17] on ultrasonic decrosslinking of XHDPE are studied.…”