Preparation of de-crosslinked polyethylene from waste crosslinked high-density polyethylene using supercritical fluids

Preparation of de-crosslinked polyethylene from waste crosslinked high-density polyethylene using supercritical fluids | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Preparation of de-crosslinked polyethylene from waste crosslinked high-density polyethylene using supercritical fluids Hang-kyu Cho, Hansang Kim This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3309384/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 24 Feb, 2024 Read the published version in Korean Journal of Chemical Engineering → Version 1 posted 5 You are reading this latest preprint version Abstract This study examined a method for de-crosslinking high-density polyethylene (HDPE) for use in heating pipes using supercritical fluids and recycling them into polyethylene. Waste crosslinked HDPE is mostly incinerated because it is a thermosetting plastic and cannot be recycled. Therefore, there is an urgent need to develop new recycling technologies for crosslinked HDPE to prevent environmental pollution. Many experiments have been conducted under various subcritical and supercritical conditions using ethanol as the supercritical solvent to recycle crosslinked HDPE. Consequently, PE can be prepared via a de-crosslinking reaction. This study evaluated the characteristics of recycled polyethylene based on the reaction conditions using Fourier transform infrared spectroscopy (FT-IR), gel permeation chromatography (GPC), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and tensile strength analyses. In this study, samples with the same chemical and crystal structures were prepared under all conditions. Crosslinked polyethylene (PEX) De-crosslinking High-density polyethylene (HDPE) Supercritical fluid Recycled polyethylene Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1. Introduction The recycling of plastic waste is important for solving global environmental problems. It creates recycled products by processing discarded waste and reusing them as a new energy source.[ 1 , 2 ] Crosslinked high-density polyethylene (HDPE) is prepared through a ram extrusion process, using a crosslinking agent. Crosslinked HDPE, also known as PEX, is widely used for carrying hot water at high temperatures and cannot be reshaped because they do not melt under heat, owing to a crosslinked network structure with a crosslinking degree of more than 90%.[ 3 – 7 ] As a result, most PEX is incinerated. This is caused by the lack of proper treatment or recycling methods, and the insoluble and infusible nature of the intramolecular crosslinked network of polymer chains. This study introduces an environmentally friendly supercritical fluid process for recycling discarded PEX.[ 8 – 11 ] A supercritical fluid is a substance in the supercritical phase region at a temperature and pressure above its critical point where distinct liquid and gas phases do not exist.[ 12 – 17 ] It is a reaction medium with various advantages including low surface tension, low viscosity, high gas diffusion rate, and excellent liquid solubility. It has been used to render harmful chemicals harmless or to treat plastic waste. Supercritical ethanol has been shown to de-crosslink PEX above the critical point of ethanol (513.9 K and 6.14 MPa).[ 18 – 21 ] This study evaluated the structural, thermal, and physical characteristics of de-crosslinked recycled polyethylene by de-crosslinking the crosslinked structure of PEX using a supercritical fluid process (reaction solvent: ethanol). 2. Experimental 2.1. Materials PEX was fabricated by chemical crosslinking of HDPE (LG Chem, XL1800) using the ram extrusion method. The PEX had a crosslinking degree of 88% and was manufactured by Dongnam Co., Ltd. The samples were crushed to particles less than 6 mm in size before the experiment. Ethanol (Daejung Chemicals & Metals, 99.5%) was used as the solvent. Xylene (Daejung Chemicals & Metals, 98.5%) and polymer additives (BASF, Irganox1150) were used to measure the degree of crosslinking. Trichlorobenzene (Merck, 99%) and butylated hydroxytoluene (Daejung Chemicals & Metals, 99.5%) were used to determine the molecular weights. 2.2. Experimental Apparatus and procedure This study was conducted under supercritical conditions by injecting ethanol into a co-rotating twin-screw extruder (screw diameter:19 mm, L/D:40) using a metering pump to test the de-crosslinking reaction of PEX. Figure 1 shows the supercritical extrusion apparatus. The PEX (raw material) was crushed to particles 6 mm or less in diameter and fed into the feeding unit of the supercritical extrusion apparatus to carry out the de-crosslinking reaction. The experiment was conducted while regulating the process temperature and rotational speed of the screws. 2.2. Analytical Methods In this study, we measured the degree of crosslinking, molecular weight, and thermal properties to examine the properties of recycled polyethylene, a product of the de-crosslinking reaction, using infrared spectroscopy. The degree of crosslinking was measured according to the ASTM D2765 standard. The crushed sample (300 mg, ≥ 30 mesh and < 60 mesh) was placed in a 120-mesh wire mesh bag. It was then added to a round flask containing 350 g xylene and 3.5 g irganox. The mixture was boiled at 1 atm and 110 ℃ for 12 h. The sample remaining in the wire mesh bag was collected and its weight was measured. The crosslinking content was calculated using the following formula: F gel = w/w 0 *100 (1) where F gel is the degree of crosslinking (%), W is the amount of sample remaining after extraction (mg), and W o is the amount of sample before extraction (mg). The molecular weight and molecular weight distribution were measured using gel permeation chromatography (GPC 150, Waters Corp.). The recycled polyethylene specimen was dissolved in a solvent of 1,2,4-trichlorobenzene mixed with 0.01% butylated hydroxytoluene at 160°C for 4 h. It was then filtered through a metal filter, and 300 mL was injected at a flow rate of 1.0 mL/min using a Mixed-B column. Infrared (IR) spectroscopy was performed using a Spectrum GX spectrometer (PerkinElmer). The recycled polyethylene film was scanned 18 times in the range of 800–4,500 cm − 1 . The thermal properties were measured using differential scanning calorimetry (DSC 2950, Du Pont). Approximately 5 mg of polyethylene were placed in an aluminum pan and heated at a rate of 10 ℃/min up to 180 ℃ under nitrogen gas to eliminate thermal history before measuring the thermal properties. X-ray diffraction analysis was performed using a Miniflex diffractometer (Rigaku). Recycled polyethylene in film form was measured at an angle of gyration (2θ) between 7° and 40°, and tensile strength specimens of de-crosslinked recycled polyethylene were prepared according to ASTM D638 using an injection molding machine ( BOY 25E, Boy). The measurements were performed at room temperature using a Universal Testing Machine 5560 (Instron). The crosshead speed was determined from the mean of seven specimens at 10 mm/min. 3. Results and discussion 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 sp 3 C–H stretching, and the 1,450 cm − 1 peak, representing CH 2 bending. This indicates that the basic structure of polyethylene did not change with the depolymerization reaction. In particular, the absence of sp 2 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. 4. Conclusions The following conclusions were obtained using a supercritical methanol treatment process for PEX resin. (1) It is possible to de-crosslink crosslinked polyethylene resin using a supercritical methanol reaction. (2) The effects of the de-crosslinking reaction temperature and screw rotational speed are significant in supercritical methanol. When the reaction temperature increased, the crosslinking reaction and physical properties could be regulated. (3) The de-crosslinking reaction in supercritical methanol did not involve any side reactions except for the breakage of some main chains. The de-crosslinked recycled polyethylene had the same chemical structure as the raw material, (un-crosslinked polyethylene resin), and showed similar physical properties. Declarations Acknowledgments This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea. [No. 2022303004020A) / No. 200153646] References H. Nishida, Polymer Journal ., 43 , 435 (2011). S. M. Hong, H. K Cho, C. M. Koo, J. H. Lee, W. Y. Park, H. S. Lee and Y. W. Lee, Korean Chemical Engineering Research ., 46 , 63 (2008). G. Brunner, Supercritical Fluid Science and Technology ., 5 , 511 (2014). H. Hirabayashi, A. Iguchi, K. Yamada, H. Nishimura, K. Ikawa and H. Honma, Materials Sciences and Applications ., 4 , 497 (2013). U. W. Gedde and M. Ifwarson, Polymer Engineering and Science ., 30 , 202, (1990). S. M. Tamboli, S. T. Mhaske and D. D. Kale, Materials for Total Joint Arthroplasty ., 11 , 133 (2015). C. M. Koo, Y. J. Cho, B. G. Cho and S. M. Hong , Journal of the Korean Institute of Resources Recycling ., 23 , 71 (2014). H. K. Cho, S. M. Hong, K. Y. Baek, H. S. Lee, Y. W. Lee and C. M, Koo, Macromolecular Research ., 17, 950 (2009). B. K. Baek, Y. H. La, A. S. Lee, H. S. Han, S. H. Kim, S. M. Hong and C. M Koo, Polymer Degradation and Stability ., 130 , 103 (2016). H. S. Lee, J. H. Jeong, S. M. Hong, C. M. Koo, H. K. Cho, and Y. W. Lee, Korean Chemical Engineering Research ., 50 , 88 (2012). G. C. Hwang, K. H. Kim, S. Y. Bae, S. C. Yi and H. Kumazawa, Korean Journal of Chemical Engineering ., 18 , 396 (2001). G. N. Sapkale, S. M. Patil, U. S. Surwase and P. K. Bhatbhage, International Journal of Chemical Science ., 8 , 729 (2010). H. S. Lee, J. H. Jeong, G. Y. Hong, H. K .Cho, B. K. Baek, C. M. Koo, S. M. Hong, J. H. Kim and Y. W. Lee, Industrial & Engineering Chemistry Research ., 52 , 6633 (2013). H. K. Cho and J. S. Lim, Korean Chemical Engineering Research ., 55 , 220 (2017). H. K. Cho and J. S. Lim, International Journal of Thermophysics ., 38 , 175 (2017). J. H. Yim, W. S. Kim and J. S. Lim, The Journal of Supercritical Fluids ., 82 , 168 (2013). D. E. Kwon, M. G. Aregay, B. K. Park and Y. W. Lee, Korean Journal of Chemical Engineering ., 38 , 2560 (2021). I. Okajima, A. Kajima, A. Katsuzaki, T. Goto, T. Yamazaki and T. Sako, Journal of chemical engineering of Japan ., 43 , 231 (2010). Y. J. Kwon, S. M. Hong and C. M. Koo, Journal of Polymer Science Part B: Polymer Physics ., 48 , 1265 (2010). S. D Yoon and H. S. Byun, Clean Technology ., 18 , 123 (2012). F. Sun, J. Guo, Y. Li, S. Bai and Q. Wang, Royal Society open science ., 6 , 1 (2019). T. Goto, S. Ashihara, T. Yamazaki, I. Okajima, T. Sako, Y. Iwamoto, M. Ishibashi and T. Sugeta, Industrial & Engineering Chemistry Research ., 50 , 5661 (2011). Cite Share Download PDF Status: Published Journal Publication published 24 Feb, 2024 Read the published version in Korean Journal of Chemical Engineering → Version 1 posted Editorial decision: Major Revisions Needed 03 Oct, 2023 Reviewers agreed at journal 05 Sep, 2023 Reviewers invited by journal 05 Sep, 2023 Editor assigned by journal 04 Sep, 2023 First submitted to journal 30 Aug, 2023 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-3309384","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":231100987,"identity":"f4b32d47-8a83-47a6-942c-24d6bbf1cd7a","order_by":0,"name":"Hang-kyu Cho","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA60lEQVRIiWNgGAWjYBACAzB5gIGBXwLMkpAhXovkDIgWHiDB2ECUFoMbEAHCWszZzx5+wXDmcOLm283PHt2oseBhkG4+/gCfFsuevDQLhhuHE7fdOWZunHMM6DCZY4n4HXYgx8z4zweglhsJZtI5bEAtEjmG+LWcf2NmwADUsnlG+jfpnH8gLfkf8Wu5kWP8AOSwDRI5ZtK5bWBbCITYjTdmDAxn0o1n3Mgpk87tk+Bhk0gznIHfYTnGHxiOWcv2z0jfJp3zrU6OXyL5wQd8WoCADRjvzUhcAspBgBloZh0R6kbBKBgFo2DEAgDMv0ywwzgHOQAAAABJRU5ErkJggg==","orcid":"","institution":"Gachon University","correspondingAuthor":true,"submittingAuthor":false,"prefix":"","firstName":"Hang-kyu","middleName":"","lastName":"Cho","suffix":""},{"id":231100988,"identity":"725adbda-582c-453d-b70f-d91e742d4877","order_by":1,"name":"Hansang Kim","email":"","orcid":"https://orcid.org/0000-0002-2531-5864","institution":"Gachon 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treatment.\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3309384/v1/bba0eaf108b95b697bed50fb.jpg"},{"id":42858270,"identity":"c36b465f-3e77-4e50-aee5-a3401374395d","added_by":"auto","created_at":"2023-09-08 20:29:02","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":31929,"visible":true,"origin":"","legend":"\u003cp\u003eGel content of PEX after supercritical methanol treatment at various screw rotation speeds.\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3309384/v1/58c52573b7cc280603da9398.jpg"},{"id":42857311,"identity":"99a5205c-3310-49cc-99d3-5139c2714bf5","added_by":"auto","created_at":"2023-09-08 20:13:02","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":53556,"visible":true,"origin":"","legend":"\u003cp\u003eGel content of PEX after supercritical methanol treatment at various reaction temperatures.\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3309384/v1/946fde09591ff8c2bf402a05.jpg"},{"id":42858099,"identity":"12689683-87e5-48f6-9431-3b8d08702c00","added_by":"auto","created_at":"2023-09-08 20:21:02","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":57124,"visible":true,"origin":"","legend":"\u003cp\u003eFT-IR curves of raw HDPE and PEX before and after supercritical methanol treatment at various temperatures.\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3309384/v1/fabdcf414509c077feb71efb.jpg"},{"id":42857312,"identity":"955b0ed4-00f6-4739-a4fb-2eaa3931ae51","added_by":"auto","created_at":"2023-09-08 20:13:02","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":50978,"visible":true,"origin":"","legend":"\u003cp\u003eDSC curves of raw HDPE and PEX before and after supercritical methanol treatment at various temperatures.\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3309384/v1/8b221c42c3266dfb5931eb98.jpg"},{"id":42857307,"identity":"0937598d-e113-493a-9ff7-9dac1f813f35","added_by":"auto","created_at":"2023-09-08 20:13:02","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":55788,"visible":true,"origin":"","legend":"\u003cp\u003eXRD curves of raw HDPE and PEX before and after supercritical methanol treatment at various temperatures.\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3309384/v1/a09af3e233da992058ec33a8.jpg"},{"id":51648565,"identity":"5bba2ae9-1111-4b59-b9f0-69ba38cfe201","added_by":"auto","created_at":"2024-02-26 15:14:15","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":489931,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3309384/v1/13d6d9d5-cb06-4c94-83fa-ee39dd3e0790.pdf"}],"financialInterests":"","formattedTitle":"Preparation of de-crosslinked polyethylene from waste crosslinked high-density polyethylene using supercritical fluids","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe recycling of plastic waste is important for solving global environmental problems. It creates recycled products by processing discarded waste and reusing them as a new energy source.[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e] Crosslinked high-density polyethylene (HDPE) is prepared through a ram extrusion process, using a crosslinking agent. Crosslinked HDPE, also known as PEX, is widely used for carrying hot water at high temperatures and cannot be reshaped because they do not melt under heat, owing to a crosslinked network structure with a crosslinking degree of more than 90%.[\u003cspan additionalcitationids=\"CR4 CR5 CR6\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e] As a result, most PEX is incinerated. This is caused by the lack of proper treatment or recycling methods, and the insoluble and infusible nature of the intramolecular crosslinked network of polymer chains. This study introduces an environmentally friendly supercritical fluid process for recycling discarded PEX.[\u003cspan additionalcitationids=\"CR9 CR10\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] A supercritical fluid is a substance in the supercritical phase region at a temperature and pressure above its critical point where distinct liquid and gas phases do not exist.[\u003cspan additionalcitationids=\"CR13 CR14 CR15 CR16\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e] It is a reaction medium with various advantages including low surface tension, low viscosity, high gas diffusion rate, and excellent liquid solubility. It has been used to render harmful chemicals harmless or to treat plastic waste. Supercritical ethanol has been shown to de-crosslink PEX above the critical point of ethanol (513.9 K and 6.14 MPa).[\u003cspan additionalcitationids=\"CR19 CR20\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e] This study evaluated the structural, thermal, and physical characteristics of de-crosslinked recycled polyethylene by de-crosslinking the crosslinked structure of PEX using a supercritical fluid process (reaction solvent: ethanol).\u003c/p\u003e"},{"header":"2. Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Materials\u003c/h2\u003e \u003cp\u003ePEX was fabricated by chemical crosslinking of HDPE (LG Chem, XL1800) using the ram extrusion method. The PEX had a crosslinking degree of 88% and was manufactured by Dongnam Co., Ltd. The samples were crushed to particles less than 6 mm in size before the experiment. Ethanol (Daejung Chemicals \u0026amp; Metals, 99.5%) was used as the solvent. Xylene (Daejung Chemicals \u0026amp; Metals, 98.5%) and polymer additives (BASF, Irganox1150) were used to measure the degree of crosslinking. Trichlorobenzene (Merck, 99%) and butylated hydroxytoluene (Daejung Chemicals \u0026amp; Metals, 99.5%) were used to determine the molecular weights.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Experimental Apparatus and procedure\u003c/h2\u003e \u003cp\u003eThis study was conducted under supercritical conditions by injecting ethanol into a co-rotating twin-screw extruder (screw diameter:19 mm, L/D:40) using a metering pump to test the de-crosslinking reaction of PEX. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the supercritical extrusion apparatus. The PEX (raw material) was crushed to particles 6 mm or less in diameter and fed into the feeding unit of the supercritical extrusion apparatus to carry out the de-crosslinking reaction. The experiment was conducted while regulating the process temperature and rotational speed of the screws.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Analytical Methods\u003c/h2\u003e \u003cp\u003eIn this study, we measured the degree of crosslinking, molecular weight, and thermal properties to examine the properties of recycled polyethylene, a product of the de-crosslinking reaction, using infrared spectroscopy.\u003c/p\u003e \u003cp\u003eThe degree of crosslinking was measured according to the ASTM D2765 standard. The crushed sample (300 mg, \u0026ge;\u0026thinsp;30 mesh and \u0026lt;\u0026thinsp;60 mesh) was placed in a 120-mesh wire mesh bag. It was then added to a round flask containing 350 g xylene and 3.5 g irganox. The mixture was boiled at 1 atm and 110 ℃ for 12 h. The sample remaining in the wire mesh bag was collected and its weight was measured. The crosslinking content was calculated using the following formula:\u003c/p\u003e \u003cp\u003eF\u003csub\u003egel\u003c/sub\u003e= w/w\u003csub\u003e0\u003c/sub\u003e*100 (1)\u003c/p\u003e \u003cp\u003ewhere F\u003csub\u003egel\u003c/sub\u003e is the degree of crosslinking (%), W is the amount of sample remaining after extraction (mg), and W\u003csub\u003eo\u003c/sub\u003e is the amount of sample before extraction (mg).\u003c/p\u003e \u003cp\u003eThe molecular weight and molecular weight distribution were measured using gel permeation chromatography (GPC 150, Waters Corp.). The recycled polyethylene specimen was dissolved in a solvent of 1,2,4-trichlorobenzene mixed with 0.01% butylated hydroxytoluene at 160\u0026deg;C for 4 h. It was then filtered through a metal filter, and 300 mL was injected at a flow rate of 1.0 mL/min using a Mixed-B column. Infrared (IR) spectroscopy was performed using a Spectrum GX spectrometer (PerkinElmer). The recycled polyethylene film was scanned 18 times in the range of 800\u0026ndash;4,500 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The thermal properties were measured using differential scanning calorimetry (DSC 2950, Du Pont). Approximately 5 mg of polyethylene were placed in an aluminum pan and heated at a rate of 10 ℃/min up to 180 ℃ under nitrogen gas to eliminate thermal history before measuring the thermal properties. X-ray diffraction analysis was performed using a Miniflex diffractometer (Rigaku). Recycled polyethylene in film form was measured at an angle of gyration (2θ) between 7\u0026deg; and 40\u0026deg;, and tensile strength specimens of de-crosslinked recycled polyethylene were prepared according to ASTM D638 using an injection molding machine ( BOY 25E, Boy). The measurements were performed at room temperature using a Universal Testing Machine 5560 (Instron). The crosshead speed was determined from the mean of seven specimens at 10 mm/min.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e 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.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCharacterization of PEX after supercritical fluid treatment with various solvents.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eReaction temperature\u003c/p\u003e \u003cp\u003e(℃)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eReaction pressure\u003c/p\u003e \u003cp\u003e(bar)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eScrew rotation speed\u003c/p\u003e \u003cp\u003e(rpm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSolvent\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eamount of solvent\u003c/p\u003e \u003cp\u003e(ml/min)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eGel content\u003c/p\u003e \u003cp\u003e(%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eMolecular Weight\u003c/p\u003e \u003cp\u003e(Mw)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eTensile strength\u003c/p\u003e \u003cp\u003e(MPa)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePEX\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e88.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e300\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e57\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e250\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e7.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e132,005\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e35.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e300\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e85\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e250\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eMethanol\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e4.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e235,839\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e38.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e300\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e91\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e250\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eEthanol\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e4.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e157,297\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e39.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e300\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e91\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e250\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAceton\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e162,431\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e37.7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e300\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e81\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e250\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eWater\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e161,535\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e34.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e300\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e84\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e250\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eWater\u0026thinsp;+\u0026thinsp;Ethanol\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e106,425\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e35.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe 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.\u003c/p\u003e \u003cp\u003eThe 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.\u003c/p\u003e \u003cp\u003eIn 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.\u003c/p\u003e \u003cp\u003eThe 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.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e 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.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e 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.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. 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.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCharacterization of raw HDPE and PEX after supercritical methanol treatment at various temperatures.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eReaction temperature\u003c/p\u003e \u003cp\u003e(℃)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eReaction pressure\u003c/p\u003e \u003cp\u003e(bar)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eScrew rotation speed\u003c/p\u003e \u003cp\u003e(rpm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSolvent\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eamount of solvent\u003c/p\u003e \u003cp\u003e(ml/min)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eGel content\u003c/p\u003e \u003cp\u003e(%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eMolecular Weight\u003c/p\u003e \u003cp\u003e(Mw)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eTensile strength\u003c/p\u003e \u003cp\u003e(MPa)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePEX\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e88.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRaw HDPE\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e261,272\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e29.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e300\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e145\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e80\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003emethanol\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e21.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e248,713\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e58.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e320\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e109\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e80\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003emethanol\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e4.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e284,356\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e40.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e340\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e80\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003emethanol\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e176,590\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e29.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e350\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e28\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e80\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003emethanol\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e143,970\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e28.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e360\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e80\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003emethanol\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e87,356\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e30.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eIt 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.\u003c/p\u003e \u003cp\u003eAt 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.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e 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 \u0026plusmn;\u0026thinsp;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 ℃\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e 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\u0026ndash;3,000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e region, representing sp\u003csup\u003e3\u003c/sup\u003e C\u0026ndash;H stretching, and the 1,450 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e peak, representing CH\u003csub\u003e2\u003c/sub\u003e bending. This indicates that the basic structure of polyethylene did not change with the depolymerization reaction. In particular, the absence of sp\u003csup\u003e2\u003c/sup\u003e C\u0026ndash;H (3,000\u0026ndash;3,300 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), sp C\u0026ndash;H (\u0026gt;\u0026thinsp;3,300 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), and C\u0026thinsp;=\u0026thinsp;C (1,600\u0026ndash;1,650 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) peaks indicates that no double or triple bonds were formed during the supercritical reaction. Peaks for C\u0026ndash;O (900\u0026ndash;1,300 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and C\u0026thinsp;=\u0026thinsp;O (1,600\u0026ndash;1,800 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) 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.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e 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\u0026deg;C, regardless of the degree of crosslinking.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e 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.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eThe following conclusions were obtained using a supercritical methanol treatment process for PEX resin.\u003c/p\u003e \u003cp\u003e(1) It is possible to de-crosslink crosslinked polyethylene resin using a supercritical methanol reaction. (2) The effects of the de-crosslinking reaction temperature and screw rotational speed are significant in supercritical methanol. When the reaction temperature increased, the crosslinking reaction and physical properties could be regulated. (3) The de-crosslinking reaction in supercritical methanol did not involve any side reactions except for the breakage of some main chains. The de-crosslinked recycled polyethylene had the same chemical structure as the raw material, (un-crosslinked polyethylene resin), and showed similar physical properties.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgments\u003c/h2\u003e \u003cp\u003eThis work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry \u0026amp; Energy (MOTIE) of the Republic of Korea. [No. 2022303004020A) / No. 200153646]\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eH. Nishida, \u003cem\u003ePolymer Journal\u003c/em\u003e., \u003cstrong\u003e43\u003c/strong\u003e, 435 (2011).\u003c/li\u003e\n\u003cli\u003eS. M. Hong, H. K Cho, C. M. Koo, J. H. Lee, W. Y. Park, H. S. Lee and Y. W. Lee, \u003cem\u003eKorean Chemical Engineering Research\u003c/em\u003e., \u003cstrong\u003e46\u003c/strong\u003e, 63 (2008).\u003c/li\u003e\n\u003cli\u003eG. 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Okajima, T. Sako, Y. Iwamoto, M. Ishibashi and T. Sugeta, \u003cem\u003eIndustrial \u0026amp; Engineering Chemistry Research\u003c/em\u003e., \u003cstrong\u003e50\u003c/strong\u003e, 5661 (2011).\u003cstrong\u003e\u003c/strong\u003e\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"korean-journal-of-chemical-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"kjce","sideBox":"Learn more about [Korean Journal of Chemical Engineering](http://link.springer.com/journal/11814)","snPcode":"11814","submissionUrl":"https://www.editorialmanager.com/kjce/default2.aspx","title":"Korean Journal of Chemical Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Subscription","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Crosslinked polyethylene (PEX), De-crosslinking, High-density polyethylene (HDPE), Supercritical fluid, Recycled polyethylene ","lastPublishedDoi":"10.21203/rs.3.rs-3309384/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3309384/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study examined a method for de-crosslinking high-density polyethylene (HDPE) for use in heating pipes using supercritical fluids and recycling them into polyethylene. Waste crosslinked HDPE is mostly incinerated because it is a thermosetting plastic and cannot be recycled. Therefore, there is an urgent need to develop new recycling technologies for crosslinked HDPE to prevent environmental pollution. Many experiments have been conducted under various subcritical and supercritical conditions using ethanol as the supercritical solvent to recycle crosslinked HDPE. Consequently, PE can be prepared via a de-crosslinking reaction. This study evaluated the characteristics of recycled polyethylene based on the reaction conditions using Fourier transform infrared spectroscopy (FT-IR), gel permeation chromatography (GPC), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and tensile strength analyses. In this study, samples with the same chemical and crystal structures were prepared under all conditions.\u003c/p\u003e","manuscriptTitle":"Preparation of de-crosslinked polyethylene from waste crosslinked high-density polyethylene using supercritical fluids","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2023-09-08 20:12:57","doi":"10.21203/rs.3.rs-3309384/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major Revisions Needed","date":"2023-10-03T05:54:42+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2023-09-05T22:29:53+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2023-09-05T14:55:38+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2023-09-04T13:25:29+00:00","index":"","fulltext":""},{"type":"submitted","content":"Korean Journal of Chemical Engineering","date":"2023-08-30T04:38:17+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"korean-journal-of-chemical-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"kjce","sideBox":"Learn more about [Korean Journal of Chemical Engineering](http://link.springer.com/journal/11814)","snPcode":"11814","submissionUrl":"https://www.editorialmanager.com/kjce/default2.aspx","title":"Korean Journal of Chemical Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Subscription","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"fc385bb2-1e36-48de-b4eb-45c8ca75e9e7","owner":[],"postedDate":"September 8th, 2023","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-02-26T15:09:47+00:00","versionOfRecord":{"articleIdentity":"rs-3309384","link":"https://doi.org/10.1007/s11814-023-00010-3","journal":{"identity":"korean-journal-of-chemical-engineering","isVorOnly":false,"title":"Korean Journal of Chemical Engineering"},"publishedOn":"2024-02-24 15:01:24","publishedOnDateReadable":"February 24th, 2024"},"versionCreatedAt":"2023-09-08 20:12:57","video":"","vorDoi":"10.1007/s11814-023-00010-3","vorDoiUrl":"https://doi.org/10.1007/s11814-023-00010-3","workflowStages":[]},"version":"v1","identity":"rs-3309384","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3309384","identity":"rs-3309384","version":["v1"]},"buildId":"WrCJVZZCHTDjtuVLN7oU0","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}