As the greatest invention of the twentieth century, plastic has been developed for nearly a hundred years, and has successfully penetrated into all aspects of our lives, bringing us great convenience. However, due to the non-degradability of plastics, plastic waste has seriously deteriorated our living environment. The effective way to solve the problem of plastic pollution is to recycle and reuse, to form a plastic recycling application system, so that the waste plastics can be reused and reuse the value of the plastic itself.
Schematic diagram of waste plastic recycling economy system
It can be seen from the schematic diagram that the waste plastic recycling economy is roughly divided into waste plastic recycling, sorting and crushing, regenerative granulation, and recycled materials processing products. The following sections will introduce these aspects.
Recycling of waste plastics
1. Two main sources of waste plastics
1 industrial source waste plastic
Mainly from the plastic product molding processing plant, when the thermoplastic resin is directly processed or processed into a product after being mixed with the ingredients, it is inevitable that there will be waste materials, scraps, scraps, scraps, test materials, etc., and waste materials that can be used as raw materials imported from all parts of China's coastal areas. Plastics, most of which are such items, are less polluting and are generally used when broken.
2 living source waste plastic
Mainly from the use and consumption of various types of plastic products, from the domestic point of view, mainly in the plastic plastic packaging plastic daily necessities in three major areas, such waste plastic recycling is difficult, because its use is large, the frequency of use, Moreover, it is highly random, and most of it is mixed with soil domestic garbage, and the composition is complicated. White pollution is mainly caused by such waste plastics.
2. Disposal of waste plastics
At present, there are several ways to deal with waste plastics at home and abroad:
1 landfill. This is the most direct, simple, and economical practice. More than 50% of waste plastics are landfilled, leaving endless hidden dangers.
2 incineration. Using a professional burner, the heat generated by the combustion of plastic is used to generate electricity.
3 chemical recovery. Use cracking or other means to recover oil and various chemical raw materials.
4 physical recycling. That is, the recycling and recycling that we advocate, the recycling rate of waste plastics is only about 10%.
Sorting, crushing and cleaning of waste plastics
1. Sorting of waste plastics
Waste plastic recycling is first sorted. The general recycling station is to sort and recycle waste plastics. The classification and recycling of waste plastics refers to the recycling of waste plastics according to the use process and composition. However, due to the complexity of waste plastics and the value of recycling, the recycling station only carries out A rough sorting. Therefore, it is necessary to perform fine separation before processing to remove impurities, raw material classification, and the like.
The sorting method is divided into manual sorting and mechanical sorting. Manual sorting is inefficient, but flexible; mechanical sorting is now more and more perfect, and applications are becoming more common. For example, magnetic separation classification is suitable for distinguishing waste plastics with different quality and color shapes; wind sorting is suitable for waste metal mixed with metal before and after crushing; and electrostatic sorting, density sorting and temperature differential selection.
2. Breaking of waste plastics
Due to the variety of plastic products and their different sizes, when recycling relatively large products, it must be broken into small pieces of certain size by some means to facilitate the next processing.
Because the properties of waste plastics of different materials are different, the type of crushing used will be different. The commonly used crushing methods mainly include crushing, crushing, grinding and shearing.
Such as rigid PVC, PS, plexiglass, phenolic resin, urea-formaldehyde resin, polyester resin and other materials are brittle plastic, brittle and brittle, subject to compression or impact, easily broken into small pieces, such plastics It is suitable to use compression or impact pulverizing equipment; while PE, PP, PA and ABS have good toughness at room temperature, it is suitable to use shearing equipment; for some low temperature brittle materials, such as soft PVC It is suitable for low temperature crushing equipment.
3. Cleaning of waste plastics
In order to remove other substances attached to the surface of the waste plastic, so that the final recycled material has higher purity and performance, the waste plastic is generally cleaned. At present, most enterprises use the traditional washing method to clean, and other substances attached to the surface are detached by stirring, and if necessary, hot water or detergent is used for cleaning.
Waste plastic cleaning is currently generally done manually or mechanically. The manual cleaning process is low in efficiency. The plastic processing plant with small scale is mainly used for manual cleaning. The mechanical cleaning efficiency is high and the adaptability is wide. For the waste plastics and particle oil stains with waste soil and dirt, some cleaning agents can be added. Cleaning.
With the advancement of technology, some companies have developed water-free cleaning equipment suitable for film and packaging waste plastics, and have been put into use. Waterless cleaning can save a lot of water, and secondary pollution will be greatly reduced, which will become an important research direction in the future.
Comparison of waterless cleaning and water cleaning
4. Dehydration of waste plastics
The washed plastic pieces need to be dehydrated, and the dehydration methods mainly include screen dehydration and centrifugal filtration dehydration. The dewatered plastic chips can be dried as needed, and the drying is usually carried out using a hot air dryer or a heater.
Recycling of waste plastics
1. Selection of granulator
At present, the main granulators on the market are single-screw extruders, twin-screw extruders and multi-stage granulators. You can choose the right granulator according to your needs.
2. Process control
Strict control of process parameters is necessary throughout the extrusion granulation process. The main process parameters include: processing temperature, processing speed, working pressure, feed rate and pelletizing speed. In the production of enterprises, important processing parameters should be monitored and recorded to ensure the stability of particle quality.
3. Impurity filtration
Even if the waste plastics are cleaned and separated, the impurity content is still high. Therefore, it is necessary to carry out filtration treatment before the plastic melt is extruded to remove residual impurities and ensure the quality of the particles.
4. Modified regeneration of waste plastics
In order to improve the processing performance of waste plastics and achieve high value of recycled materials, the recycled materials are often modified. The most applied are physical modification, including filling reinforcement modification, blend alloy modification and the like. By modifying the recycled material, the mechanical properties can be greatly improved, and the application field can be broadened, thereby increasing the value.
Processing and utilization of waste plastic recycled materials
1. How to use recycled materials
Generally, the recycling of waste plastics can be divided into two categories: simple recycling and composite recycling. Simple recycling is to recycle a single variety of waste plastic directly or after simple processing; composite recycling is based on mixing waste plastics, and then participate in the use of other ingredients, almost all thermoplastic waste plastics, even a small amount Thermoset waste plastics can be recycled and recycled.
2. Processing of recycled materials
Forming methods for waste plastic reclaimed materials include: extrusion molding, injection molding, blow molding, press forming, calendering, casting, and foam molding.
Extrusion products are continuous profiles such as tubes, rods, wires, sheets, films, wire and cable coatings, and the like. In addition, it can also be used for mixing, plasticizing, granulating, coloring, blending, etc. of plastics.
Injection molding is suitable for almost all thermoplastics. In recent years, injection molding has also been successfully used to form certain thermoset plastics. The molding cycle of injection molding is short (several seconds to several minutes), and the quality of molded products can range from several grams to several tens of kilograms, and moldings with complicated shapes and precise dimensions, with metal or non-metal inserts can be formed at one time. Therefore, the method is highly adaptable and has high production efficiency.
Blow molding is a method in which a hot parison closed in a mold is inflated into a hollow product by means of gas pressure, or a tube parison is inflated into a tube film. This method is mainly used in the manufacture of various packaging containers and tubular membranes. Polyethylene, polyvinyl chloride, polypropylene, polystyrene, thermoplastic polyester, polycarbonate, polyamide, cellulose acetate and polyacetal resins can be used for blow molding, with polyethylene being the most widely used.
Development prospects of recycling and recycling of waste plastics
In 2016, the total amount of plastic products in China was more than 77 million tons, and it is increasing every year. The amount of waste plastic will increase linearly in the future, and will soon reach the stage where the amount of waste plastic is greater than the production of plastic products. Recycling of waste plastics will be the main way to dispose of waste plastics.
If you simply dispose of waste plastics, it will increase the pollution of land resources due to excessive accumulation, seriously hinder the penetration of water and the circulation of groundwater. It will not be profitable, and will waste valuable resources. The investment in incineration is large and the equipment is depleted. And maintenance and operation costs are high, it is necessary to control the emission of combustion gases to prevent the impact of secondary pollutants on the atmospheric environment.
With the implementation of the prohibition on the import of waste plastics, the acceleration of the construction of domestic waste plastic recycling economic system, the promotion of national policies, and the rapid advancement of waste plastic recycling technology, the domestic waste plastics industry will surely develop rapidly, we will wait and see!
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