Basic knowledge and identification methods of commonly used plastics

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Basic knowledge and identification methods of commonly used plastics 1. Conventional varieties and classifications of plastics In daily life, most of the plastics that we can directly contact or perceive are conventional general-purpose plastics, which mainly include five categories: PE, PP, ABS, PVC, and PS, these five categories of plastics account for the vast majority of plastic raw materials used, and the rest can basically be classified into special plastic varieties, such as: PPS, PPO, PA, PC, POM, etc., are rarely used in daily life products and are mainly used in high-end fields such as engineering industry and national defense technology, such as automobiles, aerospace, construction, communications and other fields. Plastics are classified according to their plasticity into thermoplastics and thermoset plastics. Under normal circumstances, thermoplastic products can be recycled, but thermosetting plastics cannot. According to the optical properties of the plastic, it can be divided into transparent, translucent and opaque raw materials, such as PS, PMMA, AS, PC, etc. are transparent plastics, while most other plastics are opaque plastics. There are many ways to classify plastics, so I won’t introduce them one by one here. 2. Properties and uses of commonly used plastic varieties 1. Polyethylene: Commonly used polyethylene can be divided into low-pressure polyethylene (HDPE), high-pressure polyethylene (LDPE) and linear high-pressure polyethylene (LLDPE). Among the three, HDPE has better thermal properties, electrical properties and mechanical properties, while LDPE and LLDPE have better flexibility, impact properties, film-forming properties, etc. LDPE and LLDPE are mainly used in packaging films, agricultural films, plastic modification, etc., while HDPE is widely used in many fields such as films, pipes, and injection daily necessities. 2. Polypropylene: Relatively speaking, polypropylene has more varieties, more complex uses, and many fields. The main varieties include homopolymer polypropylene (homopp), block copolymer polypropylene (copp) and random copolymer polypropylene (rapp). According to different uses, all Polypropylene is mainly used in wire drawing, fiber, injection, BOPP film and other fields. Copolymerized polypropylene is mainly used in household appliance injection parts, modified raw materials, daily injection products, pipes, etc. Atactic polypropylene is mainly used in transparent products, high-performance products, high-performance pipes, etc. 3. Polyvinyl chloride: Due to its low cost and self-flaming properties, it is widely used in the construction field, especially sewer pipes, plastic steel doors and windows, sheets, and artificial leather. 4. Polystyrene: As a transparent raw material, it is widely used when there is a demand for transparency, such as car lampshades, daily transparent parts, transparent cups, cans, etc. 5. ABS: It is a widely used engineering plastic with outstanding physical, mechanical and thermal properties. It is widely used in household appliances, panels, masks, assemblies, accessories, etc., especially household appliances such as washing machines, air conditioners, refrigerators, fans, etc. The usage is very large. In addition, it is also widely used in plastic modification. 3. How to identify commonly used plastics 1. Density method: Examine the density of various plastics, use liquid as the medium, and check the sinking and floating of the plastic in the liquid medium to roughly identify the major categories of plastics. For example, if a piece of plastic is placed in water and floats on the water, it can be concluded that the raw material is not PVC (because the density of PVC is >1). 2. Combustion method: It mainly examines the color of the flame and the smell and smoke emitted when burning. Generally speaking, the burning flame of polyolefin raw materials is mostly blue or light blue, the smell is relatively mild and light, and the smoke is white. However, most raw materials containing benzene or chlorine tend to emit black smoke and have a strong smell after burning. In addition, PE and PP have dripping ignition phenomenon, while PVC does not drip ignition, but has self-extinguishing phenomenon. 3. Optical method: It mainly tests the transparency of raw materials. Commonly used transparent raw materials are: PS, PC, PMMA, AS ； Translucent raw materials are: PE, random copolymer PP, homopolymer PP, soft PVC, transparent ABS, etc. Other raw materials are basically opaque. 4. Color discrimination method: Generally speaking, if the raw materials without additives contain double bonds, the color will be slightly yellow. For example, ABS will be slightly yellow due to the copolymerization of butadiene. After polymerization, the polymer still contains double bonds, so it will be slightly yellow. Most other identification methods rely on various instruments, such as infrared spectroscopy, mass spectrometry, nuclear magnetic resonance, differential thermal scanning, thermal analysis, etc.