Nowadays, all kinds of plastic products have entered thousands of households, and the plastic waste is also increasing. Various plastic bags, plastic containers, plastic toys and stationery, plastic appliance casings, vehicle bumpers, plastic pipes, industrial waste, etc. can be seen everywhere, causing great environmental pollution. The recycling of waste plastics is conducive to environmental protection and saves resources. They can be recycled and granulated, or granulated by modification, and can be used again to produce plastic products.
First, the characteristics of waste plastics
Waste plastics can be divided into three types according to their origin:
1. Corner scraps produced during the production process, which are relatively clean, less polluting and contain impurities, such as substandard film, trimming, pipe, profile part or defective product in the production of the film, injection Unfilled parts in production, etc.;
2. Used plastic waste with single material system, such as dismantled pipes, doors and windows, packaging materials or other waste plastic products that are sorted according to the type of resin;
3. Mixed waste plastics that are difficult to distinguish or cannot be separated at all, such as multi-layer co-extruded composite films, coated plastic products, composite products of plastics and Other Materials.
Different types of waste plastics have different characteristics. In terms of impurity content, the impurity content of the corner scraps in the factory production is less than 0.1%, and the scraps and other used products deposited for a certain period of time have an impurity content of 0.1%. ~0.5%. The content of composite waste plastics mixed with aluminum, cloth and paper is often greater than 10%.
For used plastic products, depending on the conditions of use, it will contain the effects of ultraviolet radiation, heat and oxygen aging, and pollutants. Different shapes of waste plastics have a large difference in the bulk density of the materials after crushing, and the bulk density of the broken materials of film, sheet and flat yarn is small, which must be considered in the process of feeding waste plastic granulation. problem.
Second, the pretreatment of waste plastics
Waste plastics from waste packaging, such as bags, shopping bags, bottles, cans, boxes and used agricultural film, are pretreated before granulation. The pretreatment process mainly includes classification, washing, crushing and drying.
The classification work is to classify a variety of waste plastic products according to the type of raw materials and the shape of the products. Sorting by raw material type requires the operator to have a good knowledge of plastics. The purpose of sorting is to avoid the incompatibility of recycled materials due to the mixing of different kinds of polymers. The performance is classified according to the shape of the product. The plastics crushing process can be carried out smoothly, because the crushing equipment used for the film, the flat yarn and the fabric thereof and the crushing equipment of some thick-walled and hard products cannot often replace each other.
There are three processes as follows for cleaning and crushing before granulation.
1. After cleaning, the large-scale waste plastic products with less serious pollution and uncomplicated structure should be cleaned first, such as automobile bumpers, instrument panels, totes, plates, etc.
First, dip with water with detergent, then rinse with water, remove and air dry. Large parts that cannot be placed in the crusher hopper due to their large volume should be coarsely crushed and then finely crushed for feeding by the extrusion granulator. In order to ensure the quality of recycled pellets, it should be dried after fine crushing. Rotary dryers with heating interlayers are often used. Superheated steam is introduced into the interlayer and rotated while being heated. The drying efficiency is high.
2. Rough washing-crushing-finishing-drying process
For contaminated profiles, used agricultural film, and packaging bags, coarse washing should be carried out first to remove foreign matter such as sand, stones and metal to prevent damage to the crusher. The waste plastic products are subjected to coarse washing, centrifugally dehydrated, and then sent to a crusher for crushing. After the crushing, it is further washed to remove the foreign matter contained therein. If the waste plastic contains oil, it can be soaked in an appropriate amount of alkaline water or warm washing liquid, and then stirred to cause friction and collision between the waste plastic blocks (sheets) to remove dirt, rinse, dehydrate and dry.
3. Mechanized cleaning
Before the waste plastic enters the cleaning equipment, it is crushed in a dry or wet crushing equipment, dried and then blown into a storage silo, and the crushed material is quantitatively input into the washing tank by a screw feeder. The two counter-rotating paddle shafts slowly transport material through the cleaning tank, and the resulting eddy current rinses off the dirt on the plastic. The dirt sinks to the bottom of the tank and is removed at the bottom of the tank at regular intervals. The cleaned waste floats and is discharged by a screw conveyor. Most of the water was removed. The screw input unit feeds the broken material into the drying system. The drying system consists of a rotary dryer and a hot air dryer. The residual moisture of the material output from the drying system accounts for 1% to 2%. The cleaned material is sent to the storage bin, which is then sent to the extrusion granulator to produce pellets.
Third, the extrusion granulation process of waste plastics
Waste plastics differ in performance from new resins in that they undergo the thermal history and shear history of the forming process and experience the effects of heat, oxygen, light, weather and various media during use. Therefore, the mechanical properties of the recycled materials, including tensile strength and impact properties are lower than the original resin, the surface structure changes caused by cracking, the appearance quality is also not as good as before, the color is yellow and the transparency is decreased.
The performance variations of various materials are different. The change in polyolefin material is relatively small. Due to the reduction of the relative molecular mass caused by processing, especially the multiple processing, it can be compensated by the crosslinking reaction, so that the processability can be kept constant to some extent; the styrene copolymer is different, and each time a process is performed, The tensile properties are reduced once. After about four processes, the toughness reduction is very serious. Moreover, the effect of the rubber phase impact modifier is also lowered due to cross-linking. Although it is high-impact polystyrene, the impact toughness is not better than that of general-purpose polystyrene.
The performance of waste plastics can be improved by blending new materials or adding specific stabilizers and additives. For example, adding antioxidants and heat stabilizers can reduce the adverse effects of heat and oxygen during the granulation process of waste plastics. In some mixed waste plastics, compatibilizers may also be added as appropriate, such as EPDM or EVA in the waste plastics mixed with polyethylene and polypropylene. Filling modification can also be carried out in waste plastic recycling granulation, such as adding 10% to 35% filler, 3% to 6% lubricant, and 6% to 4% masterbatch in the PP waste film. The recycled material prepared by using CaC O 3 as a filler is used for injection products, which can effectively shorten the molding cycle, improve the rigidity of the product, increase the heat distortion temperature, and reduce the shrinkage rate.
The lubricant improves the fluidity of the melt. Filling, reinforcement and alloying can also be carried out in the recycling of some engineering plastics. For some moisture-absorbing materials, such as PA, PET, etc., during processing, moisture will cause degradation, resulting in a decrease in relative molecular mass, a decrease in melt viscosity, and a decrease in physical properties. The moisture in the waste plastic should be removed before processing and dried thoroughly to ensure the quality of the recycled material.
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