The recycling of waste plastics is conducive to environmental protection and saves resources. Thermoplastic wastes are valuable renewable resources that can be recycled and granulated, or modified and then granulated to be used again to produce plastic products.
First, the characteristics of waste plastics Waste plastics can be divided into three types according to the occasions: one is the corner scrap produced in the production process, the waste is relatively clean, less polluting and contains impurities, such as dissimilar in film production Specification of film, trimming, waste wire in PP flat yarn production, lead material or non-conforming product in pipe and profile production, unfilled parts in injection production, etc.; one is used, single material system Plastic waste, such as dismantled pipes, doors and windows, packaging materials that are sorted by resin type or other waste plastic products; and mixed waste plastics that are difficult to distinguish or cannot be separated at all, such as multiple layers Extruded composite films, coated plastic products, composites of plastics and other materials, etc. 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 composite waste plastic mixed with aluminum, cloth and paper is often more than 10%. For used plastics, depending on the conditions of use, it will include the effects of ultraviolet radiation, heat and oxygen aging, and the effects of pollutants. For different shapes of waste plastics, the bulk density of the materials after crushing is very different, and the bulk density of the broken materials of film, sheet and flat yarn is small, which must be considered in the process of adding waste plastic granulation. The 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 plastic varieties. The purpose of sorting is to avoid inferior performance of recycled materials due to the mixing of different kinds of polymers; the classification according to the shape of the product is to facilitate the waste. 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 are often not mutually replaceable. There are three processes for cleaning and crushing before granulation.
1. After the first cleaning and crushing process Large-scale waste plastic products with less serious pollution and uncomplicated structure should be crushed before cleaning, such as automobile bumpers, instrument panels, turnover boxes and plates. 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 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 rough washing, centrifugally dehydrated, and then sent to a plastic crusher for crushing. After crushing, it is further washed to remove the impurities 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), remove the dirt, rinse, dehydrate and dry.
3. Mechanized cleaning The large-capacity machine shows a set of mechanized cleaning equipment with high production efficiency. Before the waste plastic enters the PET cleaning equipment, it is crushed in a dry or wet crushing equipment. After drying, it is 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.
Finally. Extrusion granulation process and equipment of waste plastics The waste plastics are different in performance from the new ones because they have undergone the thermal history and shearing process of the forming process and have experienced heat during use. Oxygen, light, climate and various media effects, therefore, the mechanical properties of recycled materials, including tensile strength and impact properties are lower than the original resin, cracks cause surface structure changes, the appearance quality is not as good as before, the color is yellow And transparency has declined.