In plastic recycling, utilization is beneficial to environmental protection and resource conservation. Thermoplastic waste is a valuable renewable resource. They can be recycled and granulated, or modified and granulated, and can be used to produce plastic products again.
First, the characteristics of waste plastics
According to the occasion, waste plastics can be divided into three types:
One kind is the corner waste produced in the production process, which is relatively clean, less polluted and contains impurities, such as unqualified film and trimming in film production, waste silk in PP flat wire production, imported parts or unqualified products in pipe and profile production, unfilled parts in injection molding production, etc.
One is the waste plastic waste with a single material system, such as dismantled pipes, doors and windows, packaging materials strictly classified according to resin types or other waste plastic products;
There is also a kind of mixed waste plastics that are difficult to distinguish or can't be separated at all, such as multi-layer extruded composite films, coated plastic products, composite products of plastics and other materials, etc.
Different kinds of waste plastics have different characteristics. As far as impurity content is concerned, the impurity content of leftover materials in factory production is below 0. 1%, the impurity content of leftover materials and other used products stored for a certain time is between 0.1%and 0.5%, and the impurity content of composite waste plastics mixed with aluminum, cloth and paper is often greater than 10%. For used waste plastics, according to the different conditions of use, it will include the influence of ultraviolet radiation, heat and oxygen aging and pollutants. For different shapes of waste plastics, the bulk density of crushed materials is quite different, and the bulk density of crushed materials such as films, sheets and flat wires is relatively small, which is a factor that must be considered in the feeding process of plastic recycling granulation.
Second, the pretreatment of waste plastics
Waste plastics mainly from waste packaging materials, such as packaging bags, shopping bags, bottles, cans, boxes and waste agricultural films, should be pretreated before granulation. The pretreatment process mainly includes sorting, cleaning, crushing and drying. The work of classification is to classify all kinds of waste plastic products according to the types of raw materials and the shapes of products. Classification according to the types of raw materials requires skilled knowledge of identifying plastic varieties. The purpose of classification is to avoid the incompatibility of recycled materials and their bad performance caused by the mixing of different types of polymers. The purpose of classifying products by shape is to facilitate the smooth crushing process of waste plastics, because the crushing equipment used for films, flat yarns and their fabrics can often not replace the crushing equipment for some thick-walled and hard products.
For cleaning and crushing before granulation, there are the following three processes.
1. Cleaning before crushing process
Large-scale waste plastic products with less pollution and uncomplicated structure should be cleaned first and then broken, such as car bumpers, dashboards, turnover boxes, plates, etc. Soak it in water with detergent first, then rinse it with clear water, and take it out to dry. Larger parts that cannot be put into the crusher hopper due to their large size should be coarsely crushed first, and then finely crushed by extrusion granulator. In order to ensure the quality of recycled granular materials, it should be dried after fine crushing. The rotary dryer with heating interlayer is often used, and superheated steam is introduced into the interlayer to rotate while heating, which has high drying efficiency.
2. Rough washing-crushing-fine washing-drying process
For polluted profiles, waste agricultural films and packaging bags, rough washing should be carried out first to remove foreign bodies such as sand, stones and metals to prevent damage to the crusher. After rough washing, the waste plastic products are centrifugally dehydrated and then sent to a crusher for crushing. Further cleaning after crushing to remove impurities contained therein. If the waste plastic contains oil stains, it can be soaked in alkaline water or warm cleaning solution with appropriate concentration, and then the waste plastic blocks (pieces) are stirred, rubbed and collided to remove the dirt, rinsed and dehydrated and dried.
3. Mechanized cleaning
Before the waste plastics enter the cleaning equipment, they are crushed in dry or wet crushing equipment, dried and blown into the storage bin, and then the crushed materials are quantitatively input into the cleaning tank by the screw feeder. Two counter-rotating paddle shafts slowly transport the materials through the cleaning tank, and the generated vortex washes away the dirt on the plastic. Dirt sinks to clean the bottom of the water tank and is removed from the bottom of the water tank within a specified time interval. The cleaned waste floats on the water surface and is discharged by the screw conveyor. Most of the water was removed. The screw feeder quantitatively feeds the crushed materials into the drying system. The drying system consists of rotary dryer and hot air dryer. The residual moisture content of the output material of the drying system accounts for 1% ~ 2%. The cleaned materials are sent to the storage bin, and then from this storage bin they are sent to the extrusion granulator to produce granular materials.
3. Extrusion granulation technology and equipment for waste plastics
The properties of waste plastics are different from those of new resins, because they have experienced thermal process and shearing process in the molding process, and have experienced the effects of heat, oxygen, light, climate and various media in the use process. Therefore, the mechanical properties of recycled materials, including tensile strength and impact properties, are lower than those of the original resin. Cracks lead to changes in surface structure, poor appearance quality, yellow color and reduced transparency.
The performance changes of various materials are different. The change of polyolefin materials is relatively small. The decrease of relative molecular weight caused by processing, especially multiple processing, can be compensated by cross-linking reaction, so the processing performance can remain unchanged to some extent. The situation of styrene * * * polymer is different, and the tensile properties decrease once every processing. After about four processing steps, the toughness decreases very seriously. In addition, the effectiveness of rubber phase impact modifier is also reduced due to crosslinking. Although it is high impact polystyrene, its impact toughness is not better than that of ordinary polystyrene.
The properties of waste plastics can be improved by mixing new materials or adding specific stabilizers and additives, such as adding antioxidants and heat stabilizers, which can reduce the adverse effects of heat and oxygen in the granulation process of waste plastics. In some mixed waste plastics, compatibilizers can also be added appropriately, such as EPDM or EVA in mixed waste plastics of polyethylene and polypropylene. Filling modification can also be carried out in plastic recycling granulation, such as adding 10% ~ 35% filler, 3% ~ 6% lubricant and 2% ~ 4% masterbatch to waste PP film at the same time. When recycled material made of CaCO3 as filler is used in injection molding products, it can effectively shorten the molding cycle, improve the rigidity of products, increase the thermal deformation temperature and reduce the shrinkage rate. Lubricant improves the fluidity of melt. Some engineering plastics can also be filled, reinforced and alloyed in the recycling process. For some materials that are easy to absorb moisture, such as PA and PET, water will cause degradation during processing, which will reduce the relative molecular weight, melt viscosity and physical properties. Before treatment, the water in waste plastics should be removed and fully dried to ensure the quality of recycled materials.
There are many kinds of recycling systems for different types and shapes of waste. Extrusion granulation equipment for pre-cutting films, fiber wastes and various broken materials.
Compared with the general extrusion granulation production, the extrusion granulation equipment for recycling waste plastics has the following characteristics.
1. Feeding
The bulk density of broken waste plastic products is small, especially the broken waste films and fibers. In order to ensure that this kind of materials can be accurately and fully transported to the melting zone and the granulator head, the design form of increasing the size of the feeding section can be adopted. When the bulk density of waste plastics is less than 200 g/L, forced feeding is needed, and when it is more than 200 g/L, forced feeding device is not needed. By increasing the design of the feeding section, materials that are difficult to transport, such as PP, PA and PET fiber waste, can also be recovered satisfactorily. For PA and PET, the screw heating in the feeding section can be used to improve the conveying efficiency. By grooving the drum in the feeding section of PP and fully cooling the hopper seat, the feeding and conveying performance will be greatly improved. If the material to be added is film, silk and scraps, the inlet can be opened a little wider to facilitate feeding.
The second step is plastic refining
The fact that recycled materials are mixtures with different melt flow rates, different lubricant compositions, different fillers or different types of polymers should be considered in the plasticizing process of waste plastics. Therefore, the plasticization of waste plastics should be enough to homogenize various components in the material and achieve uniform quality.
Generally speaking, when the granulation process of waste plastics is only regeneration without filling and strengthening, a single-screw extruder is used, and if the modification treatment of filling, strengthening and alloying is also carried out during the granulation process, a twin-screw extruder with good mixing effect is needed. In terms of output, twin-screw extruder is higher than single-screw extruder.
vent
Most polyolefins do not need exhaust regeneration, while hygroscopic polymers such as PA and PET need exhaust. Some unclean pollutants on waste plastics may also be volatile substances, which will produce gas during heating. The exhaust section should ensure that the residence time of molten materials is long, the melt temperature is high, the shear deformation is strong, and the melt surface area is large, so that the gas in the melt can be fully exhausted.
4. Melt filtration
The function of melt filtration is to filter out impurities in waste plastics. These impurities will greatly reduce the quality of recycled materials. Impurities will cause bubbles to burst during film blowing, broken filaments during spinning and nozzle blockage during injection molding, which will eventually lead to product quality degradation or all unqualified products.
The allowable degree of pollution depends on the required level and quality of the final product. If recycled materials are used to produce films, the impurity particles should be less than 20μm, so that the film with a thickness of 30μm will not burst. For injection molding, even if the impurity size is larger than 100μm, it is acceptable. Therefore, the choice of filter fineness must adapt to the quality requirements or the use of secondary raw materials. Too coarse filtration is not good for quality, and too fine filtration affects economic benefits. Fine screen not only has low output, but also frequently changes screens. Otherwise, productivity will decrease and energy consumption will increase.
shake the elbows
Because recycled materials are often processed with a certain proportion of new materials, if the particle size difference is too large and the shape is irregular, it will cause uneven feeding of new and old materials, and ultimately lead to uneven product performance. Therefore, the recycled materials are granulated on the surface of the water-cooled mold, and the shape and size of the obtained particles are the least different from those of the new materials, and they are most easily mixed with the new materials.