In this study, we developed a continuous process using extruders in a pilot plant for recycling silane cross-linked polyethylene (silane-XLPE) via chemical reaction in supercritical alcohol. As a first step, an autoclave was employed to reveal the effects of temperature and pressure on the reaction that selectively decomposes the siloxane cross-linking bonds. Selective decomposition of the siloxane bond was found to occur in the supercritical alcohol. In the next step, an extruder was used to continuously feed silane-XLPE into a tube reactor containing supercritical alcohol. Silane-XLPE was then extruded with an injection of supercritical alcohol. Alcohol remained in the supercritical state inside the cylinders of the extruder and the tube reactor. The recycled polyethylene (PE) was continuously extruded from the reactor at a rate of 14 kg/h. The product produced by this continuous process was the same as that from the autoclave. The mechanical properties of the recycled PE satisfied the requirements for use as a wire and cable insulation material. These results suggest that the extruder proved to be useful for the continuous denaturation of a cross-linked polymer in supercritical alcohol.

The development of a continuous process for supercritical fluids is desirable for high-pressure technology. In a previous study, it was revealed that a twin-screw extruder was applicable as the reactor and feeder for industrial supercritical fluid processes (Goto, T.; et al. Ind. Eng. Chem. Res. 2011, 50, 5661–5666). The advantages of twin-screw extruders include their conveying ability and flexibility of design, which make them ideal for research on the applications of extruder. However, twin-screw extruders are expensive because of their complicated gearbox, cylinder shape, screw, etc. Here we report that a single-screw extruder, which is less expensive and simpler than a twin-screw extruder, is also applicable as the reactor and feeder for commercial supercritical fluid processes. Moreover, the equipment for a single-screw extruder is more compact than that of a twin-screw extruder. The results of this study will promote the application of extruders in supercritical fluid engineering research and contribute to the industrialization of a wide range of supercritical fluid techniques.

The waste of cross-linked polyethylene is burned as fuel or buried in the ground. But material recycling is more and more important for the environment in these days. But the material recycling of the cross-linked polyethylene is difficult because the processability is too low because of the cross-linking element.In this study, we successfully obtained thermoplastic recycled polyethylene from silane cross-linked polyethylene by using chemical reaction which can decompose the cross-linking element selectively in supercritical alcohol. The chemical reaction is shown in Fig. 1. Siloxane bond was decomposed by supercritical alcohol and turn to be the alcoxy silane. The new continuous process using twin screw extruder was constructed for the development of the mass production technology for the recycling of silane cross-linked PE. The new continuous process for supercritical alcohol is shown in Fig. 2.The recycled PE was successfully made by the continuous process as shown in Fig. 3. The results of the tensile tests and the measurement of the dielectric loss and volume resistivity of the recycled PE satisfied the requirement of the cable insulation. Then we made the recycled wire made frome recycled PE. The properties of recycled PE wire show that the recycled PE can be recycled as the insulation of 600 V polyethylene wire.These results indicate that the cable to cable and wire to wire recycling of silane cross-linked polyethylene will possibly be accomplished by supercritical technology using extruder.Moreover the environmental effect of this technology was evaluated by the amount of the carbon dioxide generated from the continuous process. These results indicate that recycling method using supercritical alcohol must be useful for the reduction of the environmental influence.

No abstract