Synthetic plastics have made great contributions to promoting industrial development and improving people's living standards. In the early stages of the development of the plastics industry, the chemical stability of synthetic materials is one of the contents of great concern and focus of research. Decades have passed, and its stability has been well resolved. What is dramatic is that its stability today has become an important reason hindering its further development. "Agricultural film" is a good example. Because it is difficult to be biodegraded, it can only be used once, and the easiest way to deal with it is to burn it. This not only wastes resources, but also causes air pollution.
The recycling and utilization of waste plastic products has become a worldwide issue. Traditional landfill and incineration treatment methods are gradually being replaced by reuse, degradation or decomposition processes. The recycling of waste plastic products can not only protect the environment and save energy, but also has good economic efficiency.
The industry has three concepts for the recycling and utilization of plastic products, namely, direct utilization, conversion utilization and incineration or biological decomposition.
Conversion and utilization
As the name implies, it is used for other purposes. Contains both chemical and physical methods.
Chemical methods can be divided into two types: The first method is to use the chemical properties of polymers to convert polymers into small molecular compounds or simple compounds. For example, polyester can be converted into monomers or small molecule compounds by hydrolysis, alcoholysis or aminolysis. The hydrolysis reaction is the reverse reaction of the polyester synthesis reaction, which can be made into terephthalic acid and ethylene glycol (polyester synthetic raw material monomer); methanol alcoholysis can produce methyl terephthalate and ethylene glycol; glycol alcohol The product of the solution is bishydroxyethyl terephthalate; the alcoholysis of propylene glycol produces oligomers for the synthesis of unsaturated polyesters (for paints or inks) and alkyd resins. Another method is to use the flammability of plastics to generate heat to generate electricity after incineration. These two methods have their own advantages and disadvantages. The chemical decomposition method has a high utilization rate of resources, but the process is complicated and the cost is high, which affects the recycling of plastic products. The waste gas from incineration, such as the chlorine-containing gas produced by the burning of PVC (polyvinyl chloride film), is a component of acid rain. In addition, many plastics produce waste gas due to incomplete combustion, which is very harmful to the environment and human health. This method has a tendency to be eliminated.
Physical conversion and utilization are similar to direct utilization, except that the latter is used by the manufacturer, and the former is used by other manufacturers. The process is also crushing-pelletizing-extrusion molding (stretching) or injection molding, etc., adding other items when necessary.
The following are some examples of successful physical conversion and utilization of polystyrene foam.
Manufacture sound insulation materials
After the waste foamed plastic is crushed, it is heated by infrared radiation, and its volume is reduced to less than 1/20; then it is mixed with special cement to make a "corn candy"-like building material. The noise reduction effect of this building material is 60% on average, and the noise suppression for certain frequencies can reach more than 90%. This material has now been converted to use
As the name implies, it is used for other purposes. Contains both chemical and physical methods.
On the walls and ceilings used as soundproofing facilities for power stations.
Manufacture of lightweight roof insulation materials
Crush the waste expanded polystyrene, add expanded perlite, cement and mortar, stir and mix evenly, cure on the roof for 3 days, dry naturally, and smooth with cement mortar to form a lightweight concrete insulation layer. The material made by this method has low production cost, good heat preservation, and the house is warm in winter and cool in summer.
Wash the waste expanded polystyrene, crush it to a particle size of 0.1-0.5cm, add No. 425 Portland cement, the volume ratio of the two is 2-3:
Add an early strength agent (inorganic alkali) with a mass ratio of 1-3%, stir with water to form a slurry, add rosin soap foaming agent, and stir to form a slurry, put into the mold, release after 1 hour, and cure for 7 days in water. A 10cm thick board has good heat transfer coefficient and compressive strength, and its thermal insulation performance can reach the thermal insulation effect of a clay brick wall with a thickness of 95cm. When the roof is used, apply a layer of cement mortar to the board, maintain it with water, and then use asphalt felt as a waterproof layer.
Waste expanded polystyrene materials and solvents are made into a 5% solution, and crushed corn cob fiber is added, mixed and pressed into a shape.
Manufacture waterproof membrane
Wash and dry the waste expanded polystyrene, and dissolve it with cheap phenol or chlorohydrocarbon residue. Add toughening agent and film-forming anti-seepage and wear-resistant agent to blend into viscose. After being blown or coated on the surface of glass cloth, it is dried into roof waterproofing or pipe waterproofing membrane. The solvent can be recycled after cooling.
Manufacture metal, wooden doors and windows and glass sealants
Add 50kg of waste expanded polystyrene, 40kg of No. 200 solvent oil, and 50kg of xylene in the dissolving tank, and stir to dissolve and remove impurities to obtain a transparent polystyrene compound. Add 13kSl788 industrial grade polyvinyl alcohol into another dissolving tank, add 80kg of water, and heat to dissolve at 30-90°C. Add 1kg of dibutyl phthalate and surfactant octylphenol polyoxyethylene ether in a little cold, then add proper amount of filler bentonite to make a paste. The product contains polar substances, which can increase the bonding strength and improve the emulsification performance. Both polyvinyl alcohol and bentonite can be evenly dispersed in the paste. The finished product dries quickly, has good performance, and is particularly resistant to acid and weather.
Used as an asphalt enhancer
Adding waste foamed polystyrene particles to the molten asphalt can enhance heat resistance and waterproofing, and can be used as roof and road construction materials.
Manufacture of anti-seepage and leak-proofing agents
It is produced by using waste water foamed polystyrene as raw material, with a small amount of plasticizer, and using a hot melt process. The product plasticizes quickly, dries quickly, and has the characteristics of good sealing performance, strong adhesion, acid and alkali resistance, and aging resistance. Its waterproof and anticorrosive performance greatly exceeds the commonly used plastic ointment and asphalt ointment, and the service life can reach more than 20 years.
Manufacture multifunctional resin glue
The multifunctional resin glue produced from waste polystyrene foam has the characteristics of good adhesion, high gloss, strong impact resistance, acid and alkali resistance, etc., which can be used to produce various glass fiber reinforced plastic products, which can greatly reduce production costs; in addition, Can produce waterproof coating, satin color coating, furniture putty glue and other products.
Rusty anti-rust paint
The rusty anti-rust paint produced from waste polystyrene foam has low cost and can be directly applied to the surface of steel with certain rust. It has the triple functions of anti-permeation, anti-rust and anti-corrosion, and can also be used for surface paint of civil furniture.
The processing of plastic products is mainly a physical process, that is, the addition of additives or color master batches is basically a physical mixing process. After the plastic slices or pellets are heated and melted, the injection molded product, the injection blow molded product, or the extrusion blow molding into a film or the extrusion casting sheet stretches into the film. Examples of injection molded products are foam packaging materials (blocks), examples of injection blow molded products are various plastic packaging bottles, and examples of extrusion blow molding films are multilayer co-extruded film products such as heat shrinkable film, cling film, and extruded Examples of stretched cast films are biaxially oriented polypropylene film BOPP and biaxially oriented polyester film BOPET. Since the changes of the materials in the above process are all physical changes, their chemical structure remains unchanged, which provides convenience for the application of its scraps. Therefore, in plastic products production plants, the scraps in production are generally crushed, extruded and pelletized, or melted and then extruded and pelletized, and then put into the production line for continued use. Because of this, crushers and pelletizers are essential equipment in the plastics industry. However, this method has a limitation, that is, the plastic products should be single-component or the physical properties of each component have little difference, which does not affect the secondary physical processing. For example, according to a US company, PET beer bottles made of EVOH as a sandwich have a barrier effect that can reach the level of glass bottles, and they can be completely recycled. However, for most multi-layer composite films or multi-layer co-extruded films, the physical and chemical properties of the materials of each layer are quite different and cannot be directly recycled.
Problems and solutions
Regardless of whether it is chemical or physical conversion and utilization, they are faced with a major difficulty, that is, the recycled waste plastics are mixed and difficult to identify and classify. On the other hand, they are particularly unclean (disposable polystyrene lunch boxes), and cleaning is very difficult. The cost is high, so the manufacturers are not motivated.
There are currently two main solutions: limiting the production of multi-component plastic products and improving the level of classified collection and sorting. For example, in order to promote the recycling of resources in Japan, the "Plastic Bottle Recycling Promotion Council" composed of Japanese beverage manufacturers and plastic bottle manufacturers recently decided to stop the production of colored plastic bottles that hinder recycling. The used plastic bottles will be collected in accordance with the "Container Packaging Recycling Law", and after sorting and crushing in the factory, they will be made into fiber products, hangers, trash cans, etc. In the past, in order to protect the juice and other beverages in the bottle from ultraviolet rays, manufacturers used blue and green plastic bottles. However, during recycling, if colored plastic bottles are mixed in, not only the quality of recycled products will be reduced, but manual processing is also very laborious. After all the transparent bottles are used, the solution to ultraviolet radiation is to cover all the transparent bottles with labels, and leave small holes like sewing machine needles on the labels for easy peeling off. Bottle caps made of metal such as aluminum will also be replaced by plastics made of the same material as the bottle body.
In addition, sorting is an important factor that determines the economics of material reuse. At present, foreign sorting technology has developed rapidly, and many automated sorting lines have been put into use, which can sort different types of materials very accurately. The biggest problem comes from composite plastic products.
Since plastic products are not easy to age and are difficult to be decomposed by microorganisms, so-called white pollution is caused. Instead of passive processing, it is better to find a way from the source, and because of this, the plastics industry attaches great importance to the research and development of biodegradable plastics. At present, there are almost no varieties that are satisfactory in terms of performance and biodegradability. To truly realize industrialization, there is still a long way to go.