3 Recycling of waste plastics
The use of landfills and incineration to dispose of used plastics has played a role. However, in recent years, the problem of waste resource utilization has received worldwide attention. How to turn harmful waste (waste plastic) into an effective resource has become a hot research topic in the world. The use of landfills and incineration methods will result in a certain amount of waste of resources. Therefore, people have developed new technologies for the recycling of used plastics so as to make full use of them and give full play to all the uses and values ​​of plastics.
3.1 The direct use of waste plastics
The direct use of waste plastics means that it is not necessary to carry out various types of modification, and the waste plastics are cleaned, broken, plasticized, processed and formed directly, or made into useful products by simple processing with other substances. Both domestic and foreign countries have conducted extensive research on this technology, and products have been widely used in agriculture, fisheries, construction, industry and daily necessities [6]. For example, the waste hard polyurethane foam is finely ground and added to a manually prepared cleaning paste to make an abrasive; the waste thermosetting plastic is crushed and ground into a fine material, and then the proportion of 15% and 30% is used as a filler. Added to the new resin, the resulting product has no significant change in physicochemical properties; waste soft polyurethane foam broken into pieces of the required size, can be used as a packing buffer and carpet lining material; rough, fine leather with plastic polyurethane Adhesive bonding can be continuously processed into plate [7]; waste plastics can be crushed and granulated to be used as ironmaking raw materials instead of traditional coke, which can significantly reduce carbon dioxide emissions [8].
3.2 Modification of Waste Plastics The main advantages of using direct recycling of waste plastics are the simple process and low cost of recycled products. The disadvantage is that the mechanical properties of recycled products decline greatly, and it is not appropriate to produce high-grade products. In order to improve the basic mechanical properties of waste plastic recycling materials and meet the quality requirements of special products, the researchers adopted various modification methods to modify the used plastics to meet or exceed the performance of the original plastic products. There are two commonly used modification methods: one is physical modification, and the other is chemical modification.
3.2.1 Physical Modification The physical modification of waste plastics includes the following aspects.
(1) Filling modification of activated inorganic particles: The addition of activated inorganic particles to waste thermoplastics can reduce the cost of plastic products and increase the temperature performance, but the amount must be appropriate and treated with a surfactant with good performance. [9].
(2) Toughening Modification of Waste Plastics: Toughening modification is usually performed using elastomers or blended thermoplastic elastomers with flexible chains, such as blending or copolymerizing polymers with rubber, thermoplastics, thermosetting resins, etc. . In recent years, rigid particle toughening modification has emerged, mainly including rigid organic particles and rigid inorganic particles. Commonly used rigid organic particles are polymethyl methacrylate (PMMA), polystyrene (PS), etc. The commonly used rigid inorganic particles are CaCO3, BaSO4, etc. [10].
(3) Enhanced Modification: The use of fibers for reinforcement and modification is a development hotspot in the field of polymer composites. It can modify general-purpose resins into engineering plastics and structural materials. Recycled thermoplastics (such as PP, PVC, PE, etc.) can be reinforced with fibers to increase strength and modulus over the original resin. Fiber-reinforced modification has great development prospects, and has broadened the way of recycling waste plastics.
(4) Alloying of recycled plastics: Two or more polymers are blended in a molten state, and the new material formed is a polymer alloy, which consists of simple blending, graft modification, and compatibilization. Reactive compatibilization, interpenetrating network aggregation and other methods [11]. Alloying is a hot spot in the plastics industry and an important way to improve polymer properties.
3.2.2 Chemical modification
Chemical modification refers to the introduction of other chains and functional groups in the molecular chain by grafting, copolymerization, etc., or cross-linking by cross-linking agents, or modification by nucleating agents and blowing agents. Waste plastics are given high impact resistance, excellent heat resistance, aging resistance, etc. for recycling. At present, more research work has been carried out in this area in China. Liao Bing [12] prepared cement water-reducing enhancers using waste polystyrene plastics. He added the dried polystyrene plastic to the reaction vessel, added the solvent and modifier to react at 100°C for 5 hours, dissolved it with water, neutralized and filtered with calcium hydroxide, and made it to be 10% efficient. Modified waste polystyrene plastic water reducing enhancer. Zhang Xianghe et al.[13] used waste thermoplastics, mass ratio of waste plastics, mixed solvents, gasoline, pigments, fillers, additives, modified resins, and resin-based toughening plasticizers (15-30): (50 -60): Appropriate amount: (0-45): (3-10): (0.5-5) The proportion of medium and high grade lacquer such as rust-proof, anti-corrosion paint, fluorescent paint of various colors is produced. Its excellent performance, good adhesion, strong impact resistance, the cost is about half of the regular paint, and the equipment is simple.
Guo Jinquan et al. [14] modified the characteristics of polyurethane (PU) synthetic formula and used the polyol hydroxyl of corn starch molecules to participate in the reaction of free isocyanate (NCO) during PU synthesis to modify and synthesize high-performance PU foams. The experimental results show that the material has high water absorption and does not impair the mechanical properties of the original foam. At the same time, the material has a wide range of application prospects due to its low cost. Chen Yifeng et al [15] used waste polystyrene foam as raw material, successfully modified the pellet binder by sulfonation modification, and the application results showed that this type of binder is suitable for pelletizing and compressing the wet state of the pellets. The dry, hot and hot strengths show good results and can replace conventional sodium humate, water glass, bentonite and other binders, and have a broad market prospect. The use of chemically modified methods to convert waste plastics into other valuable materials with high added value has become a hot topic in the research of waste plastic recycling technology. It is believed that more and more research results will gradually emerge in recent years.
3.3 Utilization of Waste Plastics Decomposition Products
3.3.1 Thermal Decomposition of Waste Plastics
The basic principle of the thermal decomposition technique is to perform a thorough macromolecular chain decomposition of the raw resin polymer in the waste plastic product to return it to a low molecular weight state and obtain a product with a high use value. The thermal decomposition mechanism and thermal decomposition products of different types of plastics are different [16]. The thermal decomposition of PE and PP is dominated by random chain breaking. There are almost no corresponding monomers in the thermal decomposition products; the thermal decomposition of PS is accompanied by depolymerization and random chain-reaction, and some of the thermal decomposition products contain styrene. Monomer; The thermal decomposition of PVC is preceded by the removal of hydrogen chloride, followed by chain breaking at higher temperatures to form hydrocarbon compounds. The thermal decomposition process of waste plastics can be divided into pyrolysis and catalytic low temperature decomposition. The former is generally performed at a high temperature of 600-900°C, and the latter is performed at a lower temperature of less than 450°C or even 300°C. different. Reactors used for thermal decomposition of waste plastics include: towers, furnaces, tanks, tube furnaces, fluidized beds, and extruders [2]. This technology is a more thorough recycling technology for waste plastics. The method for recovering the raw oil by high temperature pyrolysis, due to the need to carry out the reaction at a high temperature, the equipment investment is large, the recovery cost is high, and the coking phenomenon occurs during the reaction, thus limiting its application. The catalytic low-temperature decomposition, due to the reaction at a relatively low temperature, has been relatively active and has made some progress.
3.3.2 Chemical decomposition of waste plastics
Chemical decomposition refers to the process of hydrolysis or alcoholysis of waste plastics (ethanol hydrolysis, methanololysis and glycololysis, etc.). Through the decomposition reaction, plastics can be converted into monomers or low molecular weight substances, which can become polymers again. Synthetic raw materials [17]. The chemical decomposition products are even and easy to control, do not require separation and purification, and investment in production equipment is small. However, chemical decomposition technology is not suitable for the treatment of miscellaneous waste plastics due to the high requirements on the cleanliness of the pretreatment of used plastics, the uniformity of the variety, and the reagents used for decomposition. The current chemical decomposition is mainly used for polyurethane, thermoplastic polyester, polyamide and other polar waste plastics [9].
(to be continued)
