In recent years, the average annual consumption of polystyrene plastics in China has increased by 10%. 1990 has reached 2 17000 tons. With the rapid development of electronic instruments and household appliances industry and the promotion of western development, the consumption of EPS will be increasing [1]. It is estimated that by 2005, China will need120,000 tons of polystyrene foam. Most of polystyrene foam plastics are disposable, and millions of tons of white garbage are scattered in nature, which will neither rot nor degrade and disappear on its own. On the one hand, it causes serious environmental pollution, on the other hand, it wastes valuable non-renewable resources. How to recycle waste polystyrene foam reasonably and effectively has attracted the attention of researchers all over the world, including China. Since 1970s, Japan, Western Europe and the United States have successively industrialized plastic waste. By the 1990s, the comprehensive utilization technology of plastic waste was mature, and the industrialization reached 80%. By the end of 1999, the recovery rate of plastic waste in the United States reached 50%, that in Britain reached 80% and that in Japan reached 49%. Italy not only recycles its own plastic waste, but also imports plastic waste from other European countries for recycling. China began to study the recycling technology of waste plastics in the late 1980s. After the 1990s, research became active, and the rate of technology industrialization was still very low. Only about 65,438+05% of waste plastics are recycled every year [2], and most of the rest are buried. The density of polystyrene foam is very small, only 0.02-0.04 g/cm3. So it's very big, and it will occupy a large area of land. Moreover, after the foam plastic enters the soil, it will not be degraded by microorganisms, which will make the air, moisture and nutrients in the soil unable to exchange normally. But also will gradually release some harmful substances, which will affect the normal cycle of the ecosystem and worsen the soil quality in the buried area and its surrounding Fiona Fang area. There have been many patents and research reports on the recycling of polystyrene foam, and its application technology mainly focuses on the following aspects [4-9]:
1. Used for manufacturing lightweight building materials. Using expandable polystyrene pre-foamed particles or polystyrene foam waste crushed into small pieces as the main body, adding different fillers and using different binders, various light building materials are made. For example, using broken wood as filler, cement as binder, mixing with water, and then molding into various shapes of lightweight cement partition walls, or making artificial wood; Lightweight foam board supported by iron wire can be used as wallboard, countertop or decorative board; Using expanded perlite as filler can make roof insulation board; Using soil as binder and filler, evenly mixing with the same amount of polystyrene foam particles, pressing, molding, drying and calcining, can be made into light large blocks for high-rise buildings or permeable pipes for underground sewage drainage; Waste nylon yarn can be used as filler to make light bending castable. This recycling method is simple in process, large in recoverable amount and low in investment, and it is a good recycling method, but the only drawback is that the technical added value of the product is small.
2. It is used to manufacture general polystyrene. After high temperature defoaming and cooling, polystyrene foam waste is mechanically crushed, extruded into strips, and then granulated to make universal polystyrene. The main problem of this method is that because the waste foam is not cleaned before crushing and the local temperature is too high during baking and extrusion, the appearance of recycled materials may be brown, losing the colorless and transparent characteristics of new polystyrene. Its impact resistance is also poor, and it can only be used as some low-value plastic parts, which is difficult to compare with ordinary plastics such as polyethylene, polypropylene and polyvinyl chloride products.
3. Used to regenerate expandable polystyrene. Recycling waste PS foam to make e PS or reshape foam products should be said to be the most reasonable utilization direction of waste PS foam. Because the surface of waste polystyrene foam material is slightly deteriorated due to environmental pollution, the internal properties of the original polystyrene foam are maintained, creating conditions for reuse. Only in this way can we reshape or manufacture EPS and give full play to the excellent characteristics of polystyrene foam in many aspects. There are several ways to make EPS or reshape it with waste polystyrene foam: one is solution polymerization. Dissolving waste carbonized foam material in styrene monomer, adding dispersant to make styrene solution of polystyrene suspend in water, heating and polymerizing, and then adding foaming agent to continue polymerization to prepare bead foam material. The advantage of this method is that part of styrene is replaced by waste PS, which can reduce the cost. The disadvantage is that styrene monomer is consumed, and it is difficult to prepare uniform products because of the influence of unstable PS surface cleanliness on initiator activity. Second, spheroidization and re-foaming. Generally, polystyrene is cut into cylindrical particles, suspended in dispersant aqueous solution, heated to melt and spheroidize the cylinder, then cooled, pressurized with foaming agent, cooled, filtered, washed and dried at low temperature to make EPS beads [10]. The key of this method lies in the quality of raw materials, otherwise it is difficult to ensure the quality of new EPS. Third, the method of crushing and transforming beads. Selecting suitable softener, surfactant and defoamer in liquid medium, selectively crushing large pieces of waste PS foam into spherical beads with a diameter of 4-8 mm, and adding foaming agent to form foam products. This method is simple in process, consumes less additional materials, and the physical properties of molded products are close to the original waste, with small investment and great benefits, which is worth popularizing.
4. Preparation of flame retardant by bromination modification. The molecule of polystyrene contains benzene ring structure, and the hydrogen atom on benzene ring can be replaced by electrophilic reagent. Someone cleaned and dried the recycled polystyrene foam, dissolved it in dichloromethane solution, and electrophilic substituted it with liquid bromine under the catalysis of aluminum trichloride to prepare flame retardant brominated polystyrene. The bromine content can be as high as 6%. It can be used as flame retardant for PVC, ABS, polypropylene and other plastic products. Compared with other organic flame retardants, brominated polystyrene has low dosage, good flame retardant effect, and does not release toxic carcinogens such as dioxin when burning, especially when it is used in combination with antimony trioxide, its synergistic flame retardant effect is better, and it is a flame retardant with good performance. The performance of brominated polystyrene prepared by this process is comparable to that of commercial brominated polystyrene flame retardant, and the cost is low [1 1]. However, because the amount of flame retardant itself is not very large, this method can not meet the recovery of a large number of waste PS.
5. Thermal decomposition technology. On the one hand, it can be cracked to produce styrene monomer, that is, by selecting a suitable catalyst and heating, the waste polystyrene foam can be cracked to produce styrene monomer. Under the tight supply of styrene, it is a reasonable way to prepare styrene by depolymerization of polystyrene foam waste to meet the market demand. The United States, France and Japan have also carried out a large number of experiments, but there are no reports of industrialization. The main problem of producing styrene by depolymerization is that the conversion rate of styrene is relatively low, only about 70% under good conditions, generally only about 40%, which not only affects the production cost, but also makes the cleaning and continuous operation of cracking equipment difficult [12]. On the other hand, it can be cracked into oil. A mixture of low-molecular-weight hydrocarbons can be obtained by heating foamed plastics in the absence of air or decomposing them under steam, and then the mixture can be decomposed and reformed by a catalyst to obtain a sulfur-free gasoline fraction, a kerosene fraction and some gases. 1 1 kg plastic can get 1 oil, and the rest is mainly carbon residue [13]. Japan has done a lot of research in this field, and China has made some achievements in this field, and now it is being industrialized.
6. Burn to recover energy. Because the main components of polystyrene foam are carbon and hydrogen, it can be burned, and its total calorific value (about 4600kJ/kg) is higher than that of standard coal (about 2600kJ/kg) and fuel oil (about 4400kJ/kg). Therefore, it is also an effective method to utilize its combustion heat by incineration. This method has been adopted by many developed countries with abundant funds and advanced equipment. For example, Japan Steel Pipe Company uses waste plastics instead of coke as fuel and reducing agent for ironmaking; A French air-conditioning company has developed a new process to produce steam by using the heat generated by waste plastic incineration. Steam can be used for production, saving energy. But as far as China is concerned, incineration still lags behind other methods. When plastics burn, the calorific value is large, and ordinary stoves are easy to burn out, so it is not easy to burn completely. Therefore, it is necessary to design a special combustion furnace, and the equipment maintenance cost is very high. The gas generated by combustion is easy to cause secondary pollution and needs to be treated.
7. Preparation of coating by grafting modification. Coatings are all made by adding pigments to the base material, stirring and grinding. The binder is a film-forming substance, and the polystyrene foam is composed of organic polymers. After being dissolved in solvent, it can be used as the base material of coatings with good water resistance and insulation. However, the coating based on PS has poor adhesion and film-forming property. By modifying it, adding proper amount of crosslinking agent and plasticizer to improve its film-forming property and film-forming quality, various coatings can be made. For example, Li et al. smashed waste polystyrene foam and dissolved it in xylene, added initiator, and dropwise added acrylic acid at a certain temperature for graft modification to obtain component A; Dissolving petroleum asphalt in xylene to obtain component b; Uniformly mixing water, emulsifier and auxiliary agent to obtain part C; Emulsifying the above three ingredients in an emulsifying kettle to prepare a waterproof coating. Grafting acrylic acid molecules on polystyrene molecular chain not only enhances the adhesion of coatings, but also improves the stability of emulsion. The prepared coating has good heat resistance, low temperature flexibility and bonding strength. In addition, the flexibility and adhesion of polystyrene can be improved by grafting flexible butyl acrylate on the molecular chain of rigid polystyrene, and an anticorrosive coating with good performance can be made with appropriate additives.