Table 1 shows the characterization of recycled PEX prepared by de-crosslinking using 50 wt% ethanol in water, where methanol, ethanol, acetone, and water were used as co-solvents for the selection of supercritical solvents. The degree of crosslinking of the PEX is presented for comparison. The molecular weight and tensile strength of the initial PEX could not be measured because of the high degree of crosslinking.
Table 1
Characterization of PEX after supercritical fluid treatment with various solvents.
Reaction temperature (℃) | Reaction pressure (bar) | Screw rotation speed (rpm) | Solvent | amount of solvent (ml/min) | Gel content (%) | Molecular Weight (Mw) | Tensile strength (MPa) |
PEX | - | - | - | - | 88.0 | - | |
300 | 57 | 250 | - | - | 7.2 | 132,005 | 35.8 |
300 | 85 | 250 | Methanol | 5 | 4.4 | 235,839 | 38.5 |
300 | 91 | 250 | Ethanol | 5 | 4.2 | 157,297 | 39.1 |
300 | 91 | 250 | Aceton | 5 | 3.7 | 162,431 | 37.7 |
300 | 81 | 250 | Water | 5 | 1.5 | 161,535 | 34.6 |
300 | 84 | 250 | Water + Ethanol | 5 | 2.3 | 106,425 | 35.1 |
The experiment was conducted at a reaction temperature of 300 ℃, a screw rotation speed of 250 rpm, and a solvent volume of 5ml/min while changing solvents to select the supercritical fluid solvent. It was possible to fabricate partially de-crosslinked recycled polyethylene without using a supercritical solvent using a specially designed extruder for the de-crosslinking reaction. However, the recycled polyethylene had a high degree of crosslinking (7.2%), low molecular weight, and low tensile strength. In this study, various experiments were conducted to determine the process conditions that can produce a low degree of crosslinking, high molecular weight, and high tensile strength.
The solvents of the supercritical fluid and the temperature and pressure of the critical point were as follows: methanol (512.6 K and 8.09MPa), ethanol (513.9 K and 6.14 MPa), acetone (508.1 K and 4.70 MPa), and water (647.3 K and 22.12 MPa). In the experiments, water was tested below its critical point.
In testing the effects of different solvents, the product using methanol exhibited the highest degree of crosslinking (4.4%). When water was used, the degree of crosslinking was 1.5%, and the de-crosslinking reaction was optimal. The degree of crosslinking was 2.3% in the de-crosslinking reaction using ethanol and water, indicating a co-solvent effect. However, its molecular weight and tensile strength were low.
The critical point of methanol is lower than those of ethanol, water, and acetone, and the degree of crosslinking was 4.4%, indicating an overall good de-crosslinking reaction. In addition, the tensile strength of the de-crosslinked product was high (38.5 MPa), and its molecular weight was the highest (235,839). Therefore, methanol was selected as the solvent to fabricate recycled polyethylene through a de-crosslinking reaction under various supercritical conditions, and experiments were conducted under various conditions.
Figure 2 shows the change in crosslinked content according to the screw rotation speed. The crosslinking content, molecular weight, and tensile strength decreased as the screw rotation speed increased. It was confirmed that when the rotational speed of the screw for the de-crosslinking reaction was higher, the productivity of recycled polyethylene and the efficiency of the de-crosslinking reaction were higher. Moreover, when the rotational speed of the screw increased, the movement speed of the raw material increased, the extrusion efficiency decreased, and the pressure in the extruder decreased owing to decreased pressure uniformity and energy loss. As the friction of the screw increased, the physical properties deteriorated.
Table 2 presents the molecular weight and molecular weight distribution of the crosslinked polyethylene treated with supercritical methanol in the temperature range where the crosslinking content changed significantly, as shown in Fig. 2. The molecular weight and molecular weight distribution of HDPE before crosslinking (the raw material for the crosslinked polyethylene) are also shown for comparison. The molecular weight of the de-crosslinked portion of polyethylene dissolved in high-temperature xylene decreased with increasing reaction temperature.
Table 2
Characterization of raw HDPE and PEX after supercritical methanol treatment at various temperatures.
Reaction temperature (℃) | Reaction pressure (bar) | Screw rotation speed (rpm) | Solvent | amount of solvent (ml/min) | Gel content (%) | Molecular Weight (Mw) | Tensile strength (MPa) |
PEX | - | - | - | - | 88.0 | - | |
Raw HDPE | - | - | - | - | 0 | 261,272 | 29.3 |
300 | 145 | 80 | methanol | 5 | 21.3 | 248,713 | 58.4 |
320 | 109 | 80 | methanol | 5 | 4.9 | 284,356 | 40.8 |
340 | 34 | 80 | methanol | 5 | 1.1 | 176,590 | 29.2 |
350 | 28 | 80 | methanol | 5 | 0.8 | 143,970 | 28.5 |
360 | 19 | 80 | methanol | 5 | 0 | 87,356 | 30.4 |
It was not possible to measure the molecular weight and tensile strength of the polyethylene supercritically reacted at 300 ℃ because 21.3% crosslinked content remained, and the weight-average molecular weight (Mw) and tensile strength of the de-crosslinked resin had a high degree of crosslinking.
At temperatures above 320 ℃, the PEX was mostly de-crosslinked, with little crosslinked content. At 320 ℃, the molecular weight decreased from 284,356 to 87,356. The molecular weight of the HDPE was approximately 370,807 before crosslinking. It was also confirmed that the de-crosslinked recycled polyethylene had a wider molecular weight distribution than the un-crosslinked HDPE before crosslinking.
Figure 3 shows the changes in crosslinked content with a change in reaction temperature to quantitatively determine the extent of the de-crosslinking reaction of PEX treated with supercritical methanol. In this study, the depolymerization reaction was performed while injecting methanol at 5 ml/min at a screw rotation speed of 80 rpm. The error range of the measured crosslinking content was ± 3%. The results showed that the crosslinking content decreased with increasing temperature. Noticeably, the de-crosslinking reaction hardly occurred below 280 ℃, where the crosslinked content was above 40%. The crosslinked content decreased abruptly at 300 ℃ and above, and the crosslinked content approached zero at 350 ℃
Figure 4 shows the FT-IR spectra of the regenerated polyethylene produced after de-crosslinking of the PEX resin at different temperatures. Un-crosslinked polyethylene is also shown for comparison. Regardless of the supercritical de-crosslinking reaction, all samples showed peaks in the 2,800–3,000 cm− 1 region, representing sp3 C–H stretching, and the 1,450 cm− 1 peak, representing CH2 bending. This indicates that the basic structure of polyethylene did not change with the depolymerization reaction. In particular, the absence of sp2 C–H (3,000–3,300 cm− 1), sp C–H (> 3,300 cm− 1), and C = C (1,600–1,650 cm− 1) peaks indicates that no double or triple bonds were formed during the supercritical reaction. Peaks for C–O (900–1,300 cm− 1) and C = O (1,600–1,800 cm− 1) bonds were not observed, which confirms that no oxidation reactions occurred during the de-crosslinking reaction. In other words, the de-crosslinking reaction of PEX in supercritical methanol did not cause any chemical side reactions other than de-crosslinking or breaking of some main chains.
Figure 5 shows the DSC graph of the regenerated polyethylene produced after the de-crosslinking reaction of crosslinked PEX resin at different temperatures. All results were obtained during a secondary temperature elevation to obtain the melting temperature (Tm) without a thermal history and were compared to the pre-crosslinked polyethylene. All samples had the same melting point of approximately 128°C, regardless of the degree of crosslinking.
Figure 6 shows an XRD graph of the regenerated polyethylene produced after the de-crosslinking reaction of the PEX resin at different temperatures. It was confirmed to have (110) and (200) structures similar to that of polyethylene, and the area under the curve increased when the reaction temperature was higher. This confirms that when the crosslinking content decreases due to the de-crosslinking reaction, the amorphous region becomes smaller and the crystallinity increases.