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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.

Decrosslinking of peroxide crosslinked high-density polyethylene (XHDPE) of different degrees of crosslinking by means of an ultrasonic single-screw extruder (SSE) is investigated. Barrel pressure and ultrasonic power consumption during extrusion are recorded. Swelling tests, rheological tests, infrared spectroscopy, thermal analysis and tensile tests are used to elucidate the structure-property relationship of decrosslinked XHDPE. It was found that a more intensive rupture of the crosslinked network occurs in XHDPE of a higher degree of crosslinking. Analysis based on the Horikx function shows that the type of preferential bond breakage during decrosslinking of XHDPE of various degrees of crosslinking is not determined by the bond energy alone but is also influenced by structural characteristics of the network.The activation energy of viscous flow of sols extracted from various decrosslinked XHDPEs supports the analysis based on the Horikx function. The dynamic, thermal and tensile properties of the decrosslinked XHDPE are greatly affected by the type of preferential bond breakage. A significant improvement in the processability and mechanical properties of decrosslinked 2% peroxide cured XHDPE is achieved due to the occurrence of a highly preferential breakage of crosslinks during ultrasonic decrosslinking.

The effect of screw design on decrosslinking of the crosslinked high-density polyethylene (XHDPE) by means of ultrasonic twin-screw extruder with two screw configurations is investigated. Die pressure and ultrasonic power consumption during extrusion are recorded. Swelling characteristics, rheological properties, thermal analysis, scanning electron microscopy, and tensile properties are used to investigate the structure-property relationship of decrosslinked XHDPE. It is found that the screw configuration with conveying elements and reverse conveying elements (decrosslinking screws) is an effective means to reduce the gel fraction and crosslink density of decrosslinked XHDPE and significantly improve its processibility. Rheological properties of decrosslinked XHDPE are correlated with structural changes occurring during ultrasonic decrosslinking. The presence of the highly branched sol in decrosslinked XHDPE is revealed through measurements of the activation energy for flow. Comparison of morphologies of the lamellar structure of HDPE, XHDPE, and decrosslinked XHDPE reveals that the presence of the crosslink network inhibits the lamella growth. Significant improvements in the mechanical performance of decrosslinked XHDPE are obtained by using decrosslinking screws. The molecular structure and morphology of the lamellar structure of decrosslinked XHDPE are used to explain the processing-solid-state property relationship. The measured results on the gel fraction and crosslink density are compared with those of numerical simulations. V C 2014Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40680.

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Please cite this article as: Huang K, Isayev AI, Comparison between decrosslinking of crosslinked high and low density polyethylenes via ultrasonically aided extrusion, Polymer (2015), AbstractAmong various polymer wastes, management of crosslinked plastics is a major environmental problem requiring a solution. Among various crosslinked plastics, recycling of crosslinked polyethylenes is of a great importance due to the presence of a three-dimensional network. To solve this problem, novel environmentally friendly technologies for decrosslinking of the crosslinked polymers are developed based on ultrasonically assisted single (SSE) and twin screw (TSE) extruders. In particular, decrosslinking of peroxide crosslinked high-density polyethylene (XHDPE) and low-density polyethylene (XLDPE) by means of an ultrasonic SSE and TSE is investigated. Barrel pressure, die pressure and ultrasonic power consumption during extrusion are recorded. Swelling, rheological, thermal analysis and tensile tests are used to elucidate the structure-property relationships of decrosslinked XHDPE and XLDPE. The frequency dependencies of the storage and loss moduli, complex viscosity and tangent loss of XHDPE, XLDPE and their decrosslinked networks are described by the post critical gel model with its parameters correlated with gel fraction and crosslink density. The dynamic, thermal and tensile properties of the decrosslinked XHDPE and XLDPE are greatly affected by the type of preferential bond breakage. It was found that the decrosslinking of XLDPE is more difficult than that of XHDPE. An analysis based on the Horikx function reveals a highly preferential breakage of crosslinks during decrosslinking of XHDPE. In contrast to decrosslinking of XHDPE, the presence of long-chain branching in XLDPE is found to lead to the breakage of its main chains during decrosslinking. An improvement and a reduction in mechanical properties of decrosslinked XHDPE and XLDPE are, respectively, observed in comparison with those of virgin XHDPE and XLDPE. Such differences in behavior are due to the occurrence of a highly preferential breakage of crosslinks in XHDPE and the breakage of main chains in XLDPE during ultrasonic decrosslinking. Graphic AbstractStress-strain curves of HDPE, LDPE, XHDPE and XLDPE (a) and decrosslinked XHDPE (b) and XLDPE (c) by SSE and TSE with and without ultrasonic treatment.

FIG. 3. Steady state shear viscosity as a function of shear rate of the decrosslinked XHDPE obtained by TSE using compounding and decrosslinking screws without and with ultrasonic treatment at amplitudes of 10 and 13 lm. The dotted lines represent the fittings by Eq. 1a.