Plastics Industry Technology Development Center Foundation, Dr. He Chenghan: Plastic is a type of polymer material. Its advantages include lightness, toughness, and affordability, leading to its widespread use after World War II. Initially, the focus was on its toughness and durability, making it difficult to decompose in nature. In the past, due to a lack of familiarity with plastics, most plastics were disposed of through incineration or landfill, both of which caused significant ecological damage. Since humanity recognized this problem, they have developed numerous solutions. This article will introduce these methods in an accessible and accessible manner. We will first discuss what plastic is, why it resists natural degradation, and the harms of improper plastic disposal. With this basic understanding of plastic, we will explore current treatment methods that can give plastic new life, providing a deeper understanding of plastic disposal methods. Finally, we will learn how the proper use of plastics can contribute to the value of a circular economy. Keywords: Sustainable material library, thermoplastic plastic, thermosetting plastic, physical recycling, chemical recycling ■ What is plastic? [1] What people commonly call plastic is an application of polymer materials. Monomer molecules are polymerized together using chemical synthesis methods. When the molecular weight is greater than 10,000 g/mol, it is called a polymer material. This material has the characteristics of light texture, strong toughness, low price, durability, etc., so it has experienced explosive growth after World War II, and people have developed a large number of plastics for use. Plastics can be divided into two categories: thermosetting and thermoplastic according to their different properties after heating. The structures of these two types of plastics are shown in Figure 1. Figure 1 Molecular structure of polymers [1] Thermosetting plastics (Thermosetting) are materials that are cured by heat at high temperatures. After the formation of this material, the structure is a cross-linked structure, such as a mesh shape. Its characteristics are not easy to plasticize by heat again, so it is called thermosetting. Thermoplastic plastics (Thermoplastic) are materials that are plasticized at high temperatures and solidified after cooling. The structure of this type of material is often linear, like the shape of a line. Its characteristics make it easy to plasticize and shape by heating, so it is called thermoplastic. The two types of plastics have different characteristics after being heated, which in turn affects their recyclability. In terms of general recycling, thermoplastic plastics are easier to recycle than thermosetting plastics. ■ Why is plastic not easy to decompose naturally? [1] The reason why plastic is not easy to decompose in nature is that there is an attraction between molecules that holds each other, which is called bond energy. If we want to decompose plastic, we must break this bond. Bonds can be divided into primary and secondary types according to their strength. Primary bonds, also known as chemical bonds, are usually composed of covalent bonds. This type of bond is like the connector between train cars, forming the main link structure of plastic molecules. Secondary bonds, also known as physical bonds, are usually composed of van der Waals bonds. This type of bond is like the force when two parallel train passengers hold hands together. When plastic molecules approach each other, they will gather together to form secondary side chain bonds. These two bonds form the strong structure of plastic, contributing to its strength and durability. However, they also make it difficult to decompose in the natural environment. For plastic to decompose naturally in nature, it often takes hundreds or even thousands of years. This is one of the main reasons why improper plastic disposal causes environmental harm. ■ Harmful Effects of Improper Plastic Disposal: Plastics are typically disposed of through landfills and incineration, which pollute oceans, soil, and air, ultimately causing serious damage to the Earth's ecosystem. Driven by environmental concerns, countries around the world have gradually introduced policies such as plastic restrictions, reductions, and even bans to reduce the harm plastics pose to humans. In recent years, the plastics industry has begun shifting from a linear production model to a circular one. Circular production refers to a shift from a linear process of raw materials, production, use, and disposal to a recycling model where discarded plastics are recycled and given new life. Figure 2 shows a flow chart of both linear and circular production. Figure 2 Linear production and circular production flow chart ■ So how do we deal with it? In order to achieve the recycling goal, what should we do? Currently, there are generally three ways to deal with plastics. We will introduce these methods below. 1. Mechanical method [2,3] Mechanical method, also known as physical recycling, is the most common method. This method uses methods such as classification, crushing, cleaning, granulation, and modification to make thermoplastic plastics into plastic pellets for use. The advantages are simple techniques and mature technology, but it also has disadvantages such as a. There are many types of plastics, and classification requires a lot of manpower. b. The plastic pelletizing process is prone to chain breakage and cracking, resulting in a decrease in physical properties. In order to solve these shortcomings, in recent years, some companies have developed automatic screening machines to reduce the manpower of manual sorting operations, and use modification technology to solve problems such as the decrease in physical properties of plastic chain breakage. [3] The mechanical method processing flow is shown in Figure 3. Figure 3 Mechanical processing flow The Plastics Center's Sustainable Materials Library (resident) has also collected some cases of industrial applications, such as Huamei Optical's marine waste sunglasses (sample number 1-051-B01), SKB's black fountain pen (sample number 1-066-B01), etc. Figure 4 Plastic mechanical recycling products 2. Thermal cracking Thermal cracking is a part of chemical recycling. This method can convert thermosetting or thermoplastic plastics into cracked fuel oil (40%), gas fuel (20%), carbon black (20%) and water (20%) through processes such as crushing, anaerobic cracking, condensation, oil-water separation, distillation, and condensation for reuse [4]. The thermal cracking process is shown in Figure 5. Figure 5 Flow chart of thermal cracking process [5] During the anaerobic cracking process, the temperature range can be divided into different stages: low temperature (300-500℃) with a long residence time, producing carbon black; medium temperature (500~600℃) with a short residence time, producing liquid cracking oil; high temperature (600-800℃) with a long residence time, producing gas fuel. According to the literature, the cracking oil produced by this technology contains C5-C9 (light naphtha) and C10-C16 (kerosene), with a ratio of about 80% or more, and the remaining 20% is mainly C16-C26 (heavy naphtha) [4]. This recycling method has the advantages of a. cracking oil and gas fuel can be used as fuel b. can recycle both thermoplastic and thermosetting plastics, but it also has the disadvantages of a. cracking oil is still subject to regulatory restrictions and cannot be directly sold domestically b. carbon black is not stable enough to replace tires, diving suits, etc. in large quantities. In order to solve this problem, the industry currently has some corresponding countermeasures as follows: a. Use the generated pyrolysis oil and gas fuel as fuel for the cogeneration plant; b. Carbon black is sold through multiple channels to find suitable customers for application. At present, the Sustainable Materials Library (resident) also exhibits some cases of domestic industry applications, such as Huantuo Technology's use of thermal cracking to decompose its rubber into thermal cracking oil (sample number 7-013-T31) and carbon black (sample number 7-012-T31). Figure 6 Rubber thermal cracking method to regenerate products 3. Chemical method [6] Chemical recycling, also known as chemical recycling, is a general term for a developing technology. This method is divided into purification (purification), decomposition (decomposition) and petrochemical raw material reduction (feedstock recycling) according to the petrochemical process. The advantage of the chemical method is that it can process some plastics that are difficult to process by other methods, such as composite films, degradable plastics, polymer fiber composites, etc. Its chemical recycling process is shown in Figure 7. Figure 7 Chemical recycling process flow chart. [6] Purification is the process of dissolving plastics with solvents to remove additives and impurities. The purified molecules can then be repolymerized into plastics. This method is more suitable for processing plastics produced through addition polymerization, such as PVC, PS, PE, and PP. Decomposition is the process of depolymerizing plastics into oligomers or monomers through chemical methods such as methanol decomposition, glycolysis, and hydrolysis, and then polymerizing the oligomers or monomers into plastics. This method is more suitable for processing plastics produced through condensation polymerization, such as PC, PET, PA, and PU. In China, Far Eastern Group also successfully transformed waste clothing fibers into chemical raw materials using chemical recycling methods (decomposition and purification) in 2020. These waste fibers can then be re-used in the production line to make new clothing. [7] Figure 8 Plastic chemical process (decomposition & purification) recycled products [7] Petrochemical raw material reduction (feedstock recycling) is similar to the thermal cracking method mentioned above, except that this method uses oxygen-free thermal cracking technology to first reduce plastics into petrochemical liquid oil or gas and other petrochemical raw materials, and finally through repolymerization, these petrochemical raw materials are recycled into plastics or other recycled petrochemical products. There are also such applications abroad. For example, LyondellBasell successfully used the petrochemical raw material reduction (feedstock recycling) method to successfully crack plastic waste into cracking oil through thermal cracking technology, and then separated it into C2 and C3 gases through gasification, and finally through polymerization to re-make them into PE and PP plastics for use [10]. Its chemical method (petrochemical raw material reduction) processing flow is shown in Figure 9. Figure 9 Chemical method (petrochemical raw material reduction) processing flow [10] As for the above chemical methods, the chemical method has the advantage of being able to regenerate plastics into new materials, but in terms of manufacturing, the current cost is still quite high. Therefore, in order to improve its manufacturing cost, many large domestic and foreign companies such as Far Eastern Plastics [8], BASF [9], Formosa Plastics [10], LyondellBasell [10], etc. are currently working hard to carry out this series of research and development, and have even entered small-scale trial production to reduce its manufacturing cost. ■ Summary From the above-mentioned plastic processing methods, we can see that each has its own advantages and disadvantages. Therefore, in order to make plastics properly used, we should find the most valuable processing mode for different plastic products, so as to meet the needs of both environmental protection and economic development at the same time. ■ Notes The Plastic Industry Technology Development Center, with the support of the Small and Medium Enterprises Administration of the Ministry of Economic Affairs, established the "Sustainable Materials Library" physical library and online platform (https://sml.pidc.org.tw/index_tw.php), which will be officially opened to the public in 2021. The museum collects seven major categories of materials: plastic, fiber, rubber, metal, mineral, natural, and other. It also promotes the development of a sustainable circular economy for materials in Taiwan by providing services such as business matching, material consulting, design, and prototyping. Source: 1. William D. Callister Jr., Fundamentals of Materials Science and Engineering, John Wiley & Sons, Inc., 2001, 5th ed. Ch2&4. 2. INSIDE 2019 [Green Perspective] Report: Is Plastic Recycling for Jerseys or Power Generation Better? A 3-Minute Guide to Global Plastic Recycling Technologies: https://www.inside.com.tw/article/16843-what-should-we-do-about-recycled-plastic 3. Chen Liangzhe and Huang Guanye, "Recycling Composites from Waste Plastics," Industrial Materials Magazine, Issue 383, 2018. 4. Yang Renbo and Liu Yaoren, "Introduction to Taiwan's Pyrolysis Technology for Reusing Waste Plastics into Oil," Industrial Pollution Prevention and Control, Issue 141, 2017. 5. Industrial Waste Treatment and Resource Recycling Information Network, Ministry of Economic Affairs, Bureau of Industrial Waste, Technical Data, Waste Plastic Resource Recycling Technology (Part 3) Pyrolysis Technology, https://riw.tgpf.org.tw/Tech/more?id=68 6. BPF, British Plastics Federation Report, "Chemical Recycling 101," https://www.bpf.co.uk/plastipedia/chemical-recycling-101.aspx 7. Tianxia 2021, Wu Jingfang Report: "Black Clothes Are the Hardest to Dispose of": The World's Solution for Used Clothes Recycling Hidden in a 70-Year-Old Taiwanese Chemical Fiber Giant http://news.fenc.com/news_detail.aspx?lang=zh&id=5299 8. BASF Official Website 2018 Press Release: BASF Uses Chemically Recycled Plastics to Make Products for the First Time https://www.basf.com/tw/zh/who-we-are/sustainability/management-and-instruments/circular-economy/chemcycling.html 9. Juheng.com 2021 Lin Yiru Report: Formosa Chemicals and Plastics Recycling and Reusing Abandoned Oyster Ropes into Recycled Nylon Plant to Double Capacity by the End of Next Year https://news.cnyes.com/news/id/4632885 10. Start-up of LyondellBasell’s MoReTec Plant Underlines Ambitious Plastic Waste Targets – cefic.org https://cefic.org/a-solution-provider-for-sustainability/chemical-recycling-making-plastics-circular/chemical-recycling-via-conversion-to-feedstock/start-up-of-lyondellbasells-moretec-plant-underlines-ambitious-plastic-waste-targets
