ARTICLE pubs.acs.org/IECR
Continuous Process for Recycling Silane Cross-Linked Polyethylene Using Supercritical Alcohol and Extruders Toshiharu Goto,*,† Shingo Ashihara,† Takanori Yamazaki,† Idzumi Okajima,‡ Takeshi Sako,‡ Yoshihiko Iwamoto,§ Masamichi Ishibashi,§ and Tsutomu Sugeta^ †
Hitachi Cable, Ltd., 5-1-1 Hitaka-cho, Hitachi-shi, Ibaraki-ken 319-1414, Japan Shizuoka University, 3-5-1 Johoku, Hamamatsu-shi, Shizuoka-ken 432-8561, Japan § The Japan Steel Works, Ltd., 6-1 Funakoshi-minami 1, Aki-ku Hiroshima-shi, Hiroshima-ken 736-8602, Japan ^ National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki-ken 305-8565, Japan ‡
ABSTRACT: 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.
1. INTRODUCTION Recycling of cross-linked polymers poses one of the most difficult problems in the recycling of polymer waste. Silane crosslinked polyethylene (silane-XLPE) was widely used as an insulation material for wires and cables. Most of the industrial waste silane-XLPE is buried in landfills or burned as fuel,1 because silane-XLPE is difficult to process due to its low fluidity, stemming from the strength of the cross-linking element formed by the siloxane bond. The selective decomposition of the siloxane bond was studied to produce thermoplastic recycled polyethylene (PE) that can be processed as virgin PE. The previous study showed that the siloxane bond could be selectively decomposed using supercritical methanol at 300340 C for 30 min, as shown in Figure 1.2,3 The recycled PE resulting from supercritical methanol addition was thus made sufficiently processable to be extruded as wire insulation material. These results reveal the potential for recycling silane-XLPE, either cable-to-cable or wire-to-wire.3,4 To commercialize this technique, a continuous process that can feed solid material, such as silane-XLPE, into supercritical alcohol must be developed. This is the case because supercritical alcohol can only be produced in a tightly shut vessel. Therefore, continuous feeding of a solid material into supercritical fluid is a common issue in supercritical technology. The previous study5 also revealed that an extruder could be applied as a feeder and reactor for the reaction of silane-XLPE in supercritical alcohol. The process represented in the previous study5 could not, however, separate alcohol and recycled PE from each other. Recycled PE that contains alcohol cannot be processed into pellets to supply the cable production process. Moreover, the r 2011 American Chemical Society
Figure 1. Decomposition reaction of the siloxane bond in supercritical alcohol. R = alkyl group.
alcohol contained in recycled PE causes the recycled PE to foam. In other words, both separation of the alcohol and recycled PE and processing of recycled PE into pellets are important considerations when attempting to industrialize this recycling technology. In this study, we used 1-propanol as the supercritical alcohol. The critical pressure of 1-propanol is lower than that of methanol, which means that less extreme conditions can be applied to the reaction. Moreover, 1-propanol is safer than methanol in terms of ignition point and explosion limit. This paper describes the following results: (1) The chemical reaction conditions between the siloxane bond and 1-propanol were optimized so as to verify Received: August 24, 2010 Accepted: March 8, 2011 Revised: February 22, 2011 Published: March 23, 2011 5661
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Figure 2. Procedure for making silane-XLPE.
Table 1. Concentrations of Additives in Silane-XLPE additives raw PE
concentrations (wt %) 98.9
vinyl trimethoxy silane
9.9 101
DCP (dicumylperoxide)
9.9 102
dibutyltin dilaurate
4.9 102
whether the decomposition reaction of the cross-linking element occurs only in supercritical fluid. (2) We developed a continuous process that can convert crushed silane-XLPE into purified recycled PE pellets using supercritical alcohol.
2. EXPERIMENTAL SECTION 2.1. Materials. Linear, low-density polyethylene (UBE Industries Ltd.) was used as raw PE, having a density of 920 kg/m3 and a melt flow rate (MFR) of 1.0 g per 10 min. Raw PE was mixed with additives for cross-linking using the procedure shown in Figure 2. Table 1 lists the concentrations of the additives. The raw PE and additives were fed into the extruder and mixed therein. Vinyl trimethoxy silane (Chisso Co.) was grafted to the PE by adding dicumylperoxide (DCP; Mitsui Chemicals Inc.) in the extruder at 200 C. Dibutyltin dilaurate (Sakai Chemical Industry Co., Ltd.) was also fed into the extruder to act as a catalyst for the silane condensation reaction. Silane-grafted PE produced in the extruder was extruded from the die in the form of strands and thereafter was immediately cooled in a water bath. The cooled strands were then cut into pellets by a pelletizer. The pellets had cylindrical shape, measuring 3 mm in diameter and 6 mm in length. The pellets of silane-grafted PE were then crosslinked in saturated vapor at 80 C for 24 h. The gel fraction of the silane-XLPE prepared here was 60%. To confirm the mass productivity of the process, we used silane-XLPE waste that was generated in the processing of 600 V cross-linked polyethylene insulated power cable with a vinyl sheath (600 V CV cable). Silane-XLPE waste was crushed and then sifted through a sieve with 5-mm openings. The insulation of the 600 V CV cable was processed by the extruder with a mixing catalyst and then cured with steam at 6080 C. This extrusion process does generate some waste. Because the waste contains a catalyst, cross-linking occurs but cannot complete because the waste is not cured in steam. The gel fraction of the silane-XLPE waste used here was 30%. The gel fraction was lower than that of the pellets used as the ideal model for silane-XLPE, because factory waste had not been cured in steam for cross-linking. 2.2. Optimization of Chemical Reaction Conditions. The appropriate temperature and pressure for selective decomposition of the cross-linking element in a batch process were investigated and found to be under 370 C and 12 MPa,
Figure 3. Salt bath reactor.
respectively. The reaction time for the decomposition process was 30 min, which includes the time required to raise the temperature and pressure. It took 3 min to reach the reaction temperature and pressure conditions. Figure 3 shows how the batch process was carried out using a salt bath reactor. The salt used was a mixture of 45 wt % sodium nitrate and 55 wt % potassium nitrate. The autoclave is made of SUS316 and has an inner volume of 20 mL. 1-Propanol was used as the supercritical alcohol. SilaneXLPE (0.5 g; described in section 2.1) and 1-propanol (approximately 5.0 g) were put into the reactor. The autoclave was then heated in a salt bath that kept it at the reaction temperature. A manometer attached to the autoclave was used to measure pressure. Once the reaction time had elapsed, the mixture was quenched to room temperature using water. The product and alcohol were then collected from the autoclave for analysis. 2.3. Evaluation of the Chemical Structure. The gel fraction (GF) for characterizing the degree of cross-linking was determined as follows: The initial weight of specimens (Wf) was measured before immersing the specimens in xylene at 110 C. The residue after extraction was then removed from the xylene and dried at 80 C under vacuum aspiration for more than 4 h. The weight of the dried specimens (Wa) was also measured. The gel fraction was calculated from the weight of the specimens before and after extraction via the following equation: GFð%Þ ¼ ðWa =Wf Þ 100 Molecular weight is the index of decomposition of the PE main chain and was measured by high-temperature gel permeation chromatography (GPC; using an HLC-8121GPC/HT; TOSOH Co.; column, TSKgel GMHr-H(S)HT). The specimens were dissolved in o-dichlorobenzene with 0.1% 2,6-di-t-butyl-4methylphenol at 130 C. A 0.05% solution of the specimens was then prepared. The master curves for determining molecular weight were taken from those for standard polystyrene (Tsk standard polystyrene; TOSOH Co.), and the number-average molecular weight ranged from 103 to 106. FTIR (Fourier transfer infrared resonance) was adopted to analyze the chemical structure of the siloxane bond that comprised the cross-linking element.6,7 The specimens were processed into a sheet of 0.5-mm thickness by hot pressing at 180 C. The JASCO MFT2000 was used to measure the transmission infrared spectra of these specimens. 5662
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Figure 4. Diagram of the continuous process for recycling silane-XLPE using supercritical alcohol and a twin screw extruder.
2.4. Description of the Continuous Process. The continuous process for silane-XLPE was conducted using a twin-screw extruder consisting of the four main sections depicted in Figure 4: the feeding, reaction, separation, and pelletization sections. In the feeding section, silane-XLPE pellets were fed into the extruder at a rate of 14 kg/h for the chemical reaction (ExtChem) from the hopper of the extruder. Then 10 parts alcohol per 100 parts silane-XLPE were injected into the cylinder beyond the seal zone, where silane-XLPE was pressurized because of the design of the screw, which formed a seal against the alcohol. In the reaction section, the tube reactor and pressure control valve are attached to the extruder. The tube reactor has a volume of 7000 cm3 so as to maintain a reaction time of more than 30 min at a yield rate of 14 kg/h. The tube reactor has three zones, each with a separate heater that was controlled on the basis of the temperature monitored at the wall of the tube reactor. The pressure should slide if the phase transition of alcohol occurs from the supercritical state to the gaseous state inside and around the reactor outlet, and the gaseous alcohol would escape through the PE seal as a result of the slipping of recycled PE. To avoid this problem, the pressure control valve is attached to the reactor to allow control of the speed of phase separation at the reactor outlet. The reactor was regulated at 330 C and 10 MPa to keep the alcohol in the supercritical state. The temperature was measured by a thermocouple attached to the wall of the reactor. Pressure within the reactor was measured by the manometer attached between the reactor and the pressure control valve. The recycled polyethylene and alcohol were injected into the extruder for degassing (Ext-Degas). Figure 5 shows a schematic of the device for separating the alcohol from the recycled PE in Ext-Degas. The recycled PE was extruded toward the die at 200 C. The design profile filling rate in Ext-Degas is indicated in the lower portion of Figure 5. The alcohol was degassed from recycled PE in the section between the seal zone and the point of injection of materials from Ext-Chem. The pressurized, recycled PE in the seal zone prevents gas flash from either the vent or the die. As a result, gaseous alcohol was vacuumed from the back vent, meaning that pressure in the back vent box was in the vacuum condition. The pressure in the back vent box was regulated at 0.002 MPa during instrument operation, thereby allowing recycled PE in the seal zone to successfully seal the alcohol. Under this condition, alcohol is gasified and cannot dissolve into the recycled PE. As a result, the alcohol and the recycled PE are separated from each other. The strand of recycled
Figure 5. Separation of alcohol in Ext-Degas.
PE was extruded and cooled in a water bath in the pelletization section. The strand was then cut into pellets after the water bath. Figure 6 shows an image of the entire apparatus. Ext-Chem and Ext-Degas are arranged to extrude polymer in opposite directions. Note that this arrangement saves space. 2.5. Mass Productivity Test. In this section, crushed silaneXLPE was used instead of the silane-XLPE pellets described in section 2.2. Table 2 lists the conditions for testing the mass productivity of the continuous process. The reactor temperature was regulated at 335 C; pressure was maintained at more than 5.1 MPa, the critical pressure of 1-propanol. Recycled PE was produced at a rate of 20 kg/h, with 8.5 wt % alcohol (unlike the process in which silane-XLPE was injected into Ext-Chem). Pressure stability was monitored by the manometer described in section 2.4. Specimens were taken after 3, 9, and 16 h, and named Sample No. (1), (2), and (3), respectively. Time-dependent variations in the product properties were investigated by performing a tensile test and measuring the distribution of molecular weight.
3. RESULTS AND DISCUSSION 3.1. Optimized Chemical Reaction Conditions. Figure 7 shows the gel fraction and the molecular weight of products versus decomposition temperature after 30 min at 10 MPa, which were investigated using the batch process. The gel fraction became zero at temperatures over 270 C. Molecular weight dropped at temperatures over 340 C. Figure 8 plots changes in the gel fraction versus pressure after 30 min at 320 C. The gel fraction declined under increased pressure and fell to zero at pressures over 5 MPa. These results suggest temperatures of 270340 C and pressures of more than 5 MPa to allow selective decomposition of the cross-linking element. This range coincides with the supercritical region of n-propanol, as shown in Figure 9. 3.2. Continuous Process. Recycled PE can successfully be extruded in the form of strands from Ext-Degas at a rate of 14 kg/h. Figure 10 shows the appearance of recycled PE extruded from Ext-Degas. The photo on the left shows that this process can produce thermoplastic recycled PE continuously. Silane-XLPE in the reactor entailed a residence time of 30 min when being fed into Ext-Chem at a rate of 14 kg/h. The difference in pressure 5663
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Figure 6. Apearance of the continuous process for recycling silane-XLPE by using supercritical alcohol and a twin screw extruder.
Table 2. Conditions for Testing Mass Productivity of the Continuous Process crushed silane-XLPE waste generated in manufacturing silane-XLPE
600 V CV cable
processing speed (kg/h) reaction temperature (C)
20 335
reaction pressure (MPa)
5.1
injected alcohol per PE (wt %)
8.5
operation time (h)
15
Figure 8. Changes in gel fraction of recycled PE plotted against decomposition pressure.
Figure 7. Changes in gel fraction and number average molecular weight of recycled PE against decomposition temperature.
between the inlet and the outlet of the reactor zone was less than 1 MPa for this operation. Figure 11 shows the gel fraction and the molecular weight of products as plotted against reactor temperature at 10 MPa. The gel fraction was zero at 320 C, and molecular weight began dropping at 330 C. This means that 320 C was the optimized temperature for conditions of 30 min at 10 MPa when the silaneXLPE being fed contains a 60% gel fraction. The optimized temperature fell within a very narrow range relative to that found
Figure 9. Supercritical fluid condition of 1-propanol and reaction conditions to obtain recycled PE.5
in the batch process. The most important difference between the batch process and the continuous process was the total amount of 1-propanol in the system. The alcohol content of silane-XLPE in the extruder must also be important, because the ratio of alcohol to PE was very small relative to that of the batch process. Figure 12 shows the effect of the amount of alcohol used. It was 5664
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Figure 10. Strands of recycled PE extruded into a water bath in the continuous process: (a) extruded strands; (b) cross section of a strand.
Figure 11. Gel fraction of recycled PE made by the continuous process as plotted against reaction temperature.
Figure 13. FTIR spectrum of the recycled PE.
Figure 14. Pressure fluctuation in the reactor.
Figure 12. Effect of the amount of injected 1-propanol on gel fraction.
thus revealed that a specific amount of alcohol is necessary to make the gel fraction of silane-XLPE 0%. FTIR analysis was also conducted to investigate the structure of the recycled PE produced by the continuous process. The spectrum in Figure 13 shows that the siloxane bond decomposed into alkoxysilane or hydroxysilane, the same products as those found in the batch process reported in the literature.2 These results indicate that the chemical reaction in supercritical fluid of silane-XLPE in an autoclave can be reproduced in a continuous process that uses an extruder. 3.3. Mass Productivity. This section investigates the adaptability of the continuous process to other waste products. The operational stability of the process and the properties of products are analyzed here. Pressure variations in the reactor were
Figure 15. Molecular weight distribution of recycled PE. 5665
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Table 3. Properties of Recycled PE sample no. standarda virgin PE (1) (2) (3) mechanical test tensile strength (MPa)
10
32.8
350
646
626 642 634
gel fraction (%)
0
0
Mnb(104)
4.7
4.2 4.0 4.0
Mwc/Mn
4.8
4.7 4.5 4.9
elongation at break (%)
23.6 25.2 24.9 0
0
Applications of Dielectric Materials 2003, Nagoya, Japan, June 2003; IEEE: New York, 2003; pp 12181225. (5) Goto, T.; Yamazaki, T.; Sugeta, I.; Okajima, I.; Iwamoto, I.; Kakizaki, J.; Ohtake, K.; Sako, T. Kagaku Kogaku Ronbunshu 2005, 31 (6), 411–416(in Japanese). (6) Hjertberg, T.; Palmlof, M.; Sultan, B. A. J. Appl. Polym. Sci. 1991, 42, 1185–1192. (7) Hjertberg, T.; Palmlof, M.; Sultan, B. A. J. Appl. Polym. Sci. 1991, 42, 1193–1203.
a
Japan Industrial Standard JIS C3605. b Number-average molecular weight. c Weight-average molecular weight.
monitored and used as an index of stable operation of the process. Figure 14 shows that pressure was held above the critical pressure for more than 15 h. Variations in declining pressure did not depend on time. These results suggest that this technology can satisfy the requirements for industrialization. Sample Nos. (1), (2), and (3), removed at the times indicated by the arrows in Figure 14, were then evaluated. Figure 15 shows the distribution of molecular weight. Table 3 lists the numberaverage molecular weight (Mn) and molecular weight distribution (Mw/Mn) as calculated from the molecular weight distribution given in Figure 15. Recycled PE still has molecular weight heavier than 40 000, and the difference in Mw/Mn is less than 0.5. Table 3 lists the mechanical properties of the recycled PE as evaluated by tensile test. The tensile strength and elongation of every sample tested after 15 h of operation satisfied the standards for cable insulation. These results show that selective decomposition of the cross-linking in silane-XLPE proved stable in the continuous extruder-based process.
4. CONCLUSION A continuous process using supercritical alcohol and an extruder was developed for the recycling of silane-XLPE. This process was able to continuously recycle crushed waste made up of silane-XLPE in the form of recycled PE pellets. One of the most important and common problems for the commercialization of supercritical fluid technology is known to be the lack of a continuous process for the treatment of solid material. We hope that this type of process may provide one solution to the industrialization of supercritical fluid technology. ’ AUTHOR INFORMATION Corresponding Author
*E-mail address:
[email protected].
’ ACKNOWLEDGMENT This work was funded by the New Energy and Industrial Technology Development Organization (NEDO), Japan. ’ REFERENCES (1) Maruyama, M. JECTEC News 2003, 7, 18–23(in Japanese). (2) Goto, T.; Yamazaki, T.; Okajima, I.; Sugeta, T.; Miyoshi, T.; Hayashi, S.; Ohtake, K.; Sako, T. Kobunsi Ronbunsyu 2001, 58 (12), 703–709(in Japanese). (3) Goto, T.; Yamazaki, T. Hitachi Cable Rev. 2004, 23, 24–27. (4) Goto, T., Yamazaki, T., Sugeta, I., Otake, K., Okajima, I., Sako, T. Proceedings of the 7th International Conference on Properties and 5666
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