Plastic recycling: analysis of new methods

In general, waste recycling can be divided into mechanical recycling, chemical recycling and raw material recycling.

Mechanical recycling methods <br> The mechanical recycling method is to directly discard the discarded materials and make them into plastic pellets, and then send the reconstituted pellets back to the plastic manufacturing process to make new products. The first step is to collect the plastic waste discarded by the consumer from the waste container. Through the central collection system, the spent soda bottles, detergent bottles and other household wastes are collected, and then together with other recyclable wastes, they are sent to the material recycling center, where the plastic waste and other materials are separated by machine or hand, and various Mixed plastic wastes are classified. After roughly categorizing, pack it separately according to type and ship it to a plastic recycling factory.

Most of the local remanufacturers recreated transparent and colored PET, as well as primary or colored HDPE. After the remanufacturer receives the designated plastic bale from the material recovery facility, the compacted bales are sent to the shredder and broken up to form a string of loose recyclable materials that are then fed into a dedicated shaker screen. Tiny trash and dust can fall through the screen and fall into the rubbish bin. This is the first step to remove impurities.

Afterwards, the screened recyclable plastic enters the grinding and cleaning process. First cut the material into small pieces to make the labels and other container contents fall off. This will facilitate later cleaning. After shredding, add water to soften the debris and remove impurities, then send the debris to the washing machine.

Some washers use warm water and detergents; others use room temperature water, heating the water using mechanical movements during the cleaning process. This cleaning process removes residues, dust and labels.

Followed by the use of flotation cylinders to separate plastics and impurities of different densities. HDPE plastic has a lower density than water and therefore floats on the water. Dust and denser plastics (such as PET) sink to the bottom of the tank and are later removed. The separated clean-floating debris will be dried with hot air, and then the air separator will separate the film and labels. After the pieces of plastic enter the sorter, they fall into an air stream and lighter pieces of film are blown away. Heavier pieces of plastic continue to fall.

Finally, plastic debris will melt, filter, and then squeeze into small particles. At the beginning, the plastic chips are poured into large buckets to reduce the quality difference, and the chips are then fed into the extruder.

Inside the machine is a heated cylinder with a screw conveyor inside. The chips are melted in the extruder and flow through a fine mesh screen at the end of the machine. The particles of infusible impurities cannot pass through the mesh, and the pure melt squeezes through the plate that is drilled with the pores and becomes a noodle shape. During the cooling of these plastic strips, they can be quickly cut to form pellets.

Chemical Recycling <br> Chemical recycling refers to the process of depolymerizing, dismantling polycondensation or addition polymerization to change the polymer back to monomer. Plastics that can be reconstructed in this way include polyesters (such as PET bottles of soda bottles), polyamines (such as nylon for carpets), and polyurethanes (such as foams for car seats).

For long-chain polymers, the chain can be broken chemically and thermally. If the purpose of the depolymerization is to reduce the original chemical that produced the polymer, this process is called raw material or monomer production. After decomposing the polymer into various chemical substances, the chemicals recycled as raw materials can be recovered, or various chemical substances can be used as fuel; some can also be taken as raw materials and the other can be used as fuel.

Recycling of Raw Materials <br><br><br><br><br> Recycling of raw materials involves chemical recycling, but it refers to the depolymerization of polyolefins and substituted polyolefins with heat to make them into various smaller vinyl intermediates. There is not much difference between these intermediates and gasoline or lubricating oil. In some cases, heat can be used to depolymerize the addition polymer and change back directly to the monomer with a very high recovery. For example, polystyrene can be changed back to styrene, and polymethylmethacrylate can be changed back to methyl methacrylate.

At present, there are various industrial polymerization methods that can recover recycled synthetic monomers or raw materials from plastics. There are also some methods being developed. The depolymerization process for recovering the monomer feed must be very effective. In other words, there should be a very high monomer recovery and a small amount of waste. Some plastics are particularly suitable for thermal depolymerization because of their unique chemical properties. PET and certain polyamines and polyurethanes can be depolymerized very effectively and the resulting synthetic chemicals can be used to make new plastics, without any difference from the original polymer.

Recycling Examples PET plastic bottles are the most common beverage containers, especially in developed countries where the amount of use is considerable. Japan is one example. In 1997, the production volume of PET plastic bottles in Japan had almost reached 220,000 tons, but only one-tenth of them were recycled.
As environmental protection issues have become the focus of the world, the recycling campaign has become a post-processing program that various industries must consider, prompting governments across countries to implement waste recycling programs. The Japanese government and companies are also very responsive, and the demand for PET plastic bottle recycling equipment has increased. Japan’s Tomen Group has followed the trend by introducing automatic screening machines for post-consumer plastic bottles from Austria and buying dismantling plastic bottle bales from the United States.

Tomen's automatic plastic bottle screening machine is from Criterion of Binder Company and can identify eight materials such as PET, PVC, PE, PP, PS, PA and ABS. Regardless of whether the plastic bottle has a label or if the plastic bottle has been deformed, it can classify the plastic bottle at 5 to 7 per second.

Plastic Recycling Equipment with High Efficient Venting <br><br><br><br><br> Plastic Recycling Equipment with High Efficient Venting <br> For many years, the experience of plastic recycling has taught us that conventional exhaust extrusion methods often fail to meet today's requirements for plastic recycling. For the recovery of heavily printed or heavily contaminated, high-humidity plastic waste, Erema Austria has developed a new solution: the Erema TVE extruder design, in which the melt filter is located before the screw outlet of the extruder.

This design combines the best exhaust and precision filtration. The typical weakness of conventional extruders is limited pressure stability, and new extruder designs can eliminate exhaust effects that are not adequately processed or defective.

The working principle of this machine is to use the conveyor belt to automatically feed the cutting compactor. The operator does not need to quantify the amount of material. In the cutting and compacting machine, the comminution, mixing, preheating, drying and pre-compaction of the material can be accomplished in one step. The rotating cutting tool feeds the single-screw extruder with which it is mounted directly and continuously. The extruder screw gets the material and it will be plasticized. At the end of the plasticizing zone, the melt is led out of the extruder, into a fully automatic, clean filter, filtered back into the extruder, and reached the exhaust zone.

The seal between the screw plasticizing zone and the venting zone is accomplished by means of reverse sealing threads. A small portion of the filtered melt flows through the sealing bolt in the opposite direction to the direction of flow of the extruder, thereby avoiding the accumulation of machining material at the sealing opening. By axially moving the screw, the plasticizing and venting characteristics of the screw will be changed accordingly to match the different processing requirements of different materials.

After passing through the degassing zone, the melt in the extruder is given a little extra energy by means of a short and deep discharge zone, and directly enters the downstream processing tools such as granulation device, blown film head, tube head, mold, etc. .

TVE extruder has a good exhaust function, filtered impurities are accumulated before the filter screen, and the gas in the melt is discharged through the exhaust area immediately behind it. In the past, there was still the possibility of the formation of bubbles in the melt after passing through a conventional exhaust extruder. In the new design, it is filtered before entering the exhaust zone; only fully melted material can pass through the exhaust zone. Larger materials that are not completely melted will stay in front of the filter until they are completely melted. Even if impurities cause partial blockage of the filter and a pressure spike occurs, it will not affect the exhaust function (there will be no overflow of the melt from the vent opening). At the same time, the phenomenon that the gas enters the melt mouth due to the replacement of the filter and the gas enters the melt flow can also be excluded.