Recycling peroxide-crosslinked polyethylene

21 March 2014
Avraam I. Isayev and Keyuan Huang
An ultrasonic single screw extruder produces melt-processable decrosslinked high-density polyethylene with good mechanical performance.

Among the many environmental problems humankind faces in the 21st century is how to improve environmental sustainability and manage a tremendous amount of polymer waste. Uncrosslinked thermoplastics can be easily reprocessed and reused. However, a 3D internal network prevents flow and shaping of crosslinked plastics upon heating and shearing, and until now there has been no simple way to recycle them.

Polyethylene (PE) is one of the most widely used polymers with global consumption being approximately 6.4×107 tons in 2009. High-density PE (HDPE) accounts for around 2.9×107 tons, followed by linear low-density PE and low-density PE: see Figure 1(a).1 HDPE is typically used for rotational and blow molding of storage tanks, extrusion of films and sheets, wire and cable insulation, pipes and conduits, and injection molding of various products. Most crosslinked HDPE (XHDPE) waste is used as a fuel or buried in landfills, yielding little or no economic value.


(a) World consumption of polyethylene (PE) and (b) PE film scrap.1 LDPE: Low-density PE. HDPE: High-density PE. LLDPE: Linear LDPE.

Various attempts have been made to recycle crosslinked PE by decomposing silane-crosslinked low-density polyethylene (LDPE) via supercritical alcohol,2–4 size reduction of waste crosslinked LDPE by mechanochemical milling,5,6 and by modular intermeshing co-rotating twin-screw extrusion.7 In addition, recycling power transmission cables insulated with crosslinked PE and separating them by thermo-chemical, thermo-mechanical, and microwave-mechanical means has been attempted.8 However, in most of these studies crosslinked LDPE rather than HDPE was carried out.

Over the last two decades, ultrasonic-assisted extrusion technology has been successfully developed to devulcanize various rubbers,9–12 but for XHDPE, significant overheating from the dissipation of ultrasonic energy caused severe thermal degradation and compromised the properties of the decrosslinked XHDPE.13 As a result, we have been developing better controlled ultrasonic-assisted extrusion systems.14 We are currently developing a novel ultrasonic extrusion technology to decrosslink the XHDPE to obtain a processable decrosslinked XHDPE exhibiting mechanical performance close to that of the virgin XHDPE.

The ultrasonic treatment ruptures the crosslink network of the XHDPE during extrusion such that the material flows under heat and pressure. Our system uses single-screw extrusion (SSE) based on a Killion one-inch single screw extruder (see Figure 2). The extruder is equipped with two water-cooled ultrasonic horns inserted into the barrel and connected to boosters and converters generating 20kHz waves of various amplitudes.


Schematic of (a) the ultrasonic single screw extruder and (b) the screw configuration. UCM: Union Carbide mixer.

A rotational molding grade HDPE crosslinked by 1wt% dicumyl peroxide was used in experiments. The prepared XHDPE has a gel fraction of 0.81. We crushed it into particles about 4mm in diameter and processed them in the extruder at 200°C without and with the ultrasonic treatment at various flow rates and ultrasonic amplitudes. We then characterized the XHDPE and the decrosslinked product by measuring their gel content, crosslink density, rheological properties, mechanical performance, and morphology.

Increasing the ultrasonic amplitude and flow rate reduces both the crosslink density and gel fraction (see Figure 3). The decrease in these quantities indicates a substantial rupture of the crosslink network. It should be noted that without the ultrasonic waves, torque overload in the extruder prevents extrusion of the XHDPE at a 15.1g/min flow rate.


(a) Crosslink density, ν, and (b) gel fraction, ζ, of crosslinked HDPE (XHDPE) and decrosslinked XHDPE as a function of the ultrasonic amplitude at various flow rates.

Scanning electron microscopy images reveal that the original XHDPE has a featureless cryofractured surface: see Figure 4(a). After ultrasonic extrusion, however, it has microsized gel particles, confirming substantial rupture of the crosslink network: see Figure 4(b). We also compared the frequency dependence of the complex viscosity of the XHDPE and the extruded XHDPE obtained without ultrasonic treatment and with ultrasonic treatment at an ultrasonic amplitude of 10μm (b) at various flow rates (see Figure 5). The ultrasound reduces the complex viscosity, indicating significant improvement in its processability.


Scanning electron microscopy (SEM) image of (a) the cryo-fractured surface of XHDPE and (b) ultrasonically decrosslinked XHDPE obtained at 7.5g/min flow rate and 7.5μm ultrasonic amplitude.


Frequency dependence of the absolute value of the complex viscosity, |η*|, of XHDPE and decrosslinked XHDPE obtained at various flow rates (a) without ultrasonic treatment and (b) with ultrasonic treatment at an ultrasonic amplitude 10μm.

The mechanical performance of the decrosslinked XHDPE is close to that of XHDPE (see Figure 6). The decrosslinked XHDPE (obtained at 15.1g/min flow rate and 10μm ultrasonic amplitude) has a higher Young's modulus and yield stress than XHDPE (see Table 1). The stress-at-break and strain-at-break of this decrosslinked XHDPE are only slightly lower than those of the XHDPE, showing that the ultrasonic treatment preferentially breaks crosslinks rather than main chains during decrosslinking of the XHDPE, which is an important factor for XHDPE recycling.


Stress-strain curves of XHDPE and decrosslinked XHDPE obtained at a flow rate of 15.1g/min and an ultrasonic amplitude of 10μm.

Tensile properties of XHDPE and decrosslinked XHDPE obtained at a flow rate of 15.1g/min and ultrasonic amplitude of 10μm.

Tensile propertiesXHDPEDecrosslinked XHDPE
Young's modulus, MPa890±301100±45
Yield stress, MPa20.7±0.722.8±0.5
Strain at break, %620±10565±25
Stress at break, MPa31.0±0.925.0±1.0

In conclusion, the present study indicates that ultrasonically aided SSE is a promising technology for recycling XHDPE. The resulting decrosslinked XHDPE is melt processable and has good mechanical performance. Future work will include scaling up the ultrasonic SSE by modifying conventional single-screw extruders to accommodate the ultrasonic horns in the barrel. The horn dimensions and ultrasonic power will be based on the extruder output.


Authors

Avraam I. Isayev
Department of Polymer Engineering University of Akron

Avraam I. Isayev is a distinguished professor.

Keyuan Huang
Department of Polymer Engineering University of Akron

Keyuan Huang is a PhD candidate.


References

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  14. A. I. Isayev, Single and twin screw extruders with ultrasonic horns for decrosslinking and devulcanization. PCT Application PCT/US2012/033751, filed 16 April 2012, WO/2012/142562, published 18 October 2012.

DOI:  10.2417/spepro.005351