The recycling of waste plastics is conducive to environmental protection, saving resources. Thermoplastic waste is a good value
renewable resources, they will be recycled granulation, or through the modification of the granulation, can be used again to produce plastic products.
One, the characteristics of waste plastics
Waste plastics can be divided into three types according to the occasion of its generation: one is the production process generated by the edge of the waste, this
waste is cleaner, less polluted and contains impurities, such as film production of substandard film, cut edges, PP flat
wire production of waste wire, pipes, profiles, production of lead material, part or substandard products, injection production, and the production of plastic products, such as the film, cut edges. Part or substandard products in the production of pipes and profiles, unfilled parts in the production of injection
and so on; a used plastic waste with a single material system, such as dismantled pipes, doors and windows, packaging materials or other waste plastic products that have been strictly
sorted by resin type; and another kind of mixed waste plastics that are difficult to differentiate or impossible to separate, such as multi-layer ** extruded composite films, films with coatings, and other waste plastics, which are difficult to differentiate or impossible to separate. **extruded composite film, plastic products with coatings, plastic and other materials, such as composite
composite products.
Different kinds of waste plastics have different characteristics, in terms of impurity content, factory production in the edge of the waste impurity
content is less than 0.1%, piled up for a certain period of time in the edge of the material and other used products impurity content of 0.1% ~
0.5%, mixed with aluminum, cloth and paper composite waste plastics impurity content is often greater than 10%. For used waste plastics,
depending on the conditions of use, will contain ultraviolet radiation, heat, oxygen aging effects, the impact of pollutants
. For different shapes of waste plastics, the bulk density of the crushed material has a great difference, film, sheet, flat
Filament crushed material bulk density is smaller, which is in the waste plastic recycling granulation must be considered in the process of charging.
Two, the pretreatment of waste plastics
From waste packaging, such as bags, shopping bags, bottles, cans, boxes and waste agricultural film waste plastics, in
pelletizing before going through the pretreatment. The process of pretreatment mainly includes classification, cleaning, crushing and drying.
Classification of the work is a wide variety of waste plastic products according to the type of raw materials and product shape classification. According to the raw material type
class sorting requires operators to have skilled knowledge of the identification of plastic varieties, sorting is to avoid due to different kinds of
class polymer mixing caused by the incompatibility of recycled materials and poor performance; product shape classification is to facilitate the crushing of waste plastics
crushing process can be carried out smoothly, because the film, flat wire and its fabric crushing equipment used with some thick-walled, hard products, and the crushing equipment. The crushing equipment used for film, flat wire and fabrics and some thick-walled, hard products
can't be replaced by each other's crushing equipment.
For cleaning and crushing before granulation, there are three processes.
1. First cleaning and crushing process
Large waste plastic products that are not seriously polluted and have uncomplicated structure, it is appropriate to use the first cleaning and crushing process, such as automobile
bumpers, instrument panels, crates, plates and so on. First wash with water with detergent, then rinse with water, remove
and air dry. Larger pieces that cannot be put into the crusher hopper because of their size should be coarsely crushed and then finely crushed for feeding to the extruder
out pelletizer. In order to ensure the quality of regenerated granules, fine crushing should be dried, often with a heated sandwich
Rotary dryer, the sandwich is fed with superheated steam, the side of the heat while rotating, drying efficiency is high.
2. Rough washing - crushing - washing - drying process
For the contaminated profiles, waste agricultural film, bags, should first be rough washing, to remove sand, stones and metal
Foreign objects, in order to prevent damage to the crusher. Waste plastic products after rough washing centrifugal dehydration, and then sent to the crusher broken
Crushing. After crushing, they are further washed to remove the debris trapped in them. If the waste plastic contains oil, it can be soaked in alkaline water of appropriate
concentration or warm washing liquid, and then stirred to produce friction and
collision between the waste plastic blocks (pieces) to remove dirt, rinsed and then dehydrated and dried.
3. Mechanized cleaning
Figure 4-51 shows a set of high productivity mechanized cleaning equipment.
Waste plastic into the cleaning equipment, before a dry or wet crushing equipment for crushing, drying is blown
people a storage bin, and then by the spiral feeder will be crushed material quantitative input to the cleaning tank.
Two counter-rotating slurry blade shafts slowly transport the material through the wash tank, generating an eddy current that rinses away the dirt
from the plastic. The dirt sinks to the bottom of the wash tank and is removed at the bottom of the tank at specified intervals. The cleaned waste material floats
up and is discharged by a screw conveyor. Most of the water is removed. The screw feeder rations the crushed material into the drying system. The drying
system consists of a rotary dryer and a hot air dryer. The residual moisture of the material from the drying system is 1 % to 2 %.
The cleaned material is fed into a storage bin, from which it is sent to the extruder to be pelletized.
Three, waste plastic extrusion pelletizing process and equipment
Waste plastics in the performance of the new resin is different, this is due to the molding process they have endured the thermal course of
and shear course, and in the use of the process of experiencing the role of heat, oxygen, light, climate and a variety of mediums, so the mechanical properties of the regenerated
Raw materials, including Tensile strength and impact properties are lower than the original resin, cracking caused by surface structural changes,
appearance quality is not as good as before, the color is yellow, the transparency decreases.
The changes in properties are different for various materials. The change of polyolefin material is relatively small. The reduction in relative molecular mass due to processing, especially multiple processing
, can be compensated for by cross-linking reactions, and thus processability can be
kept constant to some extent. Figures 4 -52 and 4 -53 illustrate this effect.
The situation is different with styrene*** polymers, where the tensile properties decrease with each processing step. As shown in Fig.
4 -54, the toughness degradation is very severe after about four processes. Moreover, the effectiveness of the rubber-phase impact modifier
is also reduced due to cross-linking, and the impact toughness is no better than that of general-purpose polystyrene, even though it is a high-impact polystyrene.
The performance of waste plastics can be improved by blending new materials or adding specific stabilizers and additives, such as the addition of anti-
Oxygen, heat stabilizers, can make waste plastics pelletizing process to reduce the adverse effects of heat, oxygen action. In some mixed
mixed waste plastics, can also be appropriate to add compatibilizers, such as polyethylene and polypropylene mixed waste plastics in the addition of
EP D M or EVA. In the waste plastic recycling granulation can also be filled with modified, such as in the PP waste film at the same time to add
10% to 35% of the filler, 3% to 6% of the lubricant, 2% to 4% of the masterbatch. Filler for CaC O 3 produced
recycled material for injection products, can effectively shorten the molding cycle, improve the rigidity of the product, improve the heat distortion temperature, reduce
smaller shrinkage. The lubricant, on the other hand, improves the fluidity of the melt. Some engineering plastics can also be filled
reinforced and alloyed in recycling. For some hygroscopic materials, such as PA, PET, etc., in the processing, moisture will cause
degradation, so that the relative molecular mass decreases, the melt viscosity decreases, physical properties decline. Before processing, the water in the waste plastics should be removed
and dried sufficiently to ensure the quality of recycled materials.
There are a variety of recycling systems available for different types and shapes of scrap. Figure 4 -55 shows an extrusion pelletizing plant for pre
short cut film, fibrous waste and various types of shredded material.
Compared with the general extrusion pelletizing production, the extrusion pelletizing equipment for the recycling of waste plastics is characterized by the following aspects.
1. Feeding
Scrap plastic products after crushing the bulk density of the material is small, especially waste film and fiber crushed material, in order to ensure
This material can be accurately fed and the melt zone and pelletizing head feed enough, can be used to increase the size of the feeding section design
Forms, as shown in Figure 4-56. In the figure (a) for the screw charging section is conical, while the melt and metering section is cylindrical,
(b) the charging section is a larger diameter cylindrical, followed by a conical transition section, metering section is cylindrical.
When the bulk density of waste plastics is less than 200g / L need to use forced charging, more than 200g / L do not need forced charging device
placement. Increase the design of the feeding section, for materials that are not easy to transport, such as PP, PA and PET fiber waste can also be satisfactory
Recycling process. For PA, PET can be used in the charging section of the screw heating way to improve the conveying efficiency, the PP material charging
section of the barrel grooving, and the hopper seat part of the full cooling, will greatly improve the feeding and conveying performance. If the added material
is film, silk and strip edge material, the feeding port can be opened wider to facilitate the feeding.
2. Plasticizing
The plasticizing of waste plastics should take into account the fact that the recycled material is a mixture of different melt flow rates, different lubricant compositions,
different fillers or different types of polymers, so the plasticizing of waste plastics should be sufficient
enough to make the material homogenization of the various components of the quality of the uniform.
Generally speaking, the process of granulation of waste plastics is only regeneration without filling and enhancement with a single-screw extruder, if
in the process of granulation is also filling, enhancement and alloying of the modification process, it is necessary to use a good mixing effect of the twin-screw
rod extruder. In terms of output, twin-screw extruder is higher than single-screw extruder.
3. exhaust
Most of the polyolefin regeneration without exhaust, while hygroscopic polymers, such as PA, PET, exhaust is necessary. Some
Some of the waste plastics on the pollutants are not cleaned may also be some volatile substances, the heating process will produce gas. Exhaust
Section should ensure that the molten material in this longer residence time, high melt temperature, strong shear deformation and a large melt surface
accumulation, in order to make the gas in the melt is fully stripped out.
4. Melt filtration
The role of melt filtration is to filter out the impurities in the waste plastic. These impurities will make the quality of the recycled material greatly
Decrease. Impurities can cause bubble breakage during film blowing, filament breakage during spinning, nozzle clogging during injection molding, and ultimately lead to lower quality or total failure of the product
.
The degree of contamination allowed depends on the grade and quality required in the final product.
Recycled materials used for film production should have contaminant particles of less than 20 μm, so that films up to 30 μm thick can be produced without breaking
bubbles. For injection molding, even contaminant sizes greater than 100 μm are acceptable. Therefore, the filter fineness must be chosen to suit the quality requirements or the use of secondary materials.
The filter is too coarse for quality, and too fine for economic efficiency. Fine filters in addition to low yield, and change the frequency of mesh
Complex. Otherwise, resulting in reduced productivity and increased energy consumption. Replacement of the filter interval should be more than 30 min. For thin
film production of recycled plastic pelletizing, should use a layer of coarse mesh and two layers of fine mesh; for injection molding, extrusion pipes, profiles
should be used in a layer or two layers of coarse filter. The so-called coarse mesh, refers to the mesh distance of 500 μm, wire diameter of 0.37 m m over
filter; fine mesh refers to the mesh distance of 70 μm, wire diameter of 0.05 m m filter.
5. Granulation
Because the recycled material is often with a certain proportion of new material with the processing together, if the particle size is too large a difference in shape
Irregularity will cause the old and the new material is not balanced, and ultimately lead to uneven product performance. Therefore, the recycled material will be
water-cooled mold surface cutting granule, the shape and size of the granule with the smallest difference between the new material, the easiest with the new material mixed evenly.
The end of my answer I hope to be able to help you.