Briefly describe PVC recycling methods and technologies

The demand for polyvinyl chloride (PVC) in China is increasing at a rate of 10% every year. The recycling of PVC can not only solve environmental protection problems, but also alleviate the pressure of resource shortage. Especially today when the price of PVC raw materials continues to rise, its recycling and reuse is of great significance.

The construction industry uses a large amount of PVC. If waste products can be recycled, the environmental pollution caused by PVC can be greatly reduced.

In fact, PVC waste is often mixed with other plastic materials. Generally, plastics need to be sorted first, and then comprehensively utilized using physical or chemical means.

Sorting of PVC

Sorting is a difficult part of the plastic waste treatment process, and it is also one of the key factors that determine the outcome of the treatment.

Common plastics include polyvinyl chloride, polystyrene, polypropylene, low (high) density polyethylene, polyamine, polyurethane, etc. Manual sorting has many disadvantages: low efficiency; workers are in a harmful environment; errors in material classification are large, etc.

Automated sorting systems have been mostly used in industrial production, and developed countries in the world have done more research in this area. A German chemical technology association once invented the technology of using infrared rays to screen PVC from waste plastics, but the objects were limited to transparent bottles, which had the characteristics of low screening accuracy and high cost.

Japan has successfully developed technology for high-precision screening of polyvinyl chloride resin (PVC) from waste plastics. This technology uses the difference in heating characteristics (within 50°C) and charging characteristics between PVC and other plastics for detection. The detected PVC is marked with red light and can be easily screened by the operator. This method is more accurate than near-infrared technology and costs only 1/2 of it.

The Center for Plastic Recycling (CPRR) of the United States has invented a method to separate PVC bottles from other waste containers. Key to the process is the use of an X-ray fluorescence detector developed by Asoma Instruments. CPRR researchers have developed a computer system. It uses an X-ray from a radioactive isotope that excites all the electrons in the compound. When the chlorine-containing molecule is irradiated, it emits an X-ray reflection image that is easily displayed by an X-ray fluorescence analyzer. The separation work is completed by a set of mechanical devices. It can either remove all bottles containing chlorine atoms via a conveyor belt, or remove bottles that do not contain chlorine atoms.

PVC recycling technology

There are three main technologies currently used to recycle PVC:

One is physical recycling;

The second is the comprehensive utilization of chemical recycling methods;

The third is to recover and utilize energy by burning PVC materials.

Among them, the comprehensive utilization of physical methods and chemical methods is a method worthy of praise. They can recycle waste products and give full play to the performance of materials.

physical recycling method

Physical recycling refers to the recycling process of PVC waste only by mechanical processing, mainly through chopping, screening, grinding and other procedures, and finally obtains films, powders, granules or other forms of recycled materials. These recycled materials can be used to produce heavy packaging bags, agricultural water pipes and shoe soles.

PVC plastic products are divided into two types: soft products and hard products. Soft products mainly include films, cable sheaths, plastic shoes and leather products; hard products mainly include bottles, pipes, special-shaped materials, and sheets.

Among soft products, agricultural film is the most widely used category. Agricultural films are made of PVC, PE, EVA and other materials. When recycling, PVC films should be distinguished from films made of other materials. Agricultural films often contain soil, sand, grass roots, nails, etc. These impurities need to be removed and cleaned, and then dried and shredded into fragments or pellets, which can then be made into agricultural product packaging films, agricultural recycling films, etc. Water pipes and shoe soles, etc. Since the repeatedly recycled waste agricultural PVC film has been subjected to multiple heating processes and mechanical effects, it will inevitably undergo partial degradation and its strength will decrease. In order to ensure the quality of the product, some new PVC resin and corresponding additives need to be added. If the recycled material is to be made into a secondary plastic film through internal mixing, two-roller or four-roller winding, etc., and different colorants are added during the internal mixing process, the calendered film produced will also show different colors. In addition, about 30% of the plasticizer (mainly DOP) remaining in waste agricultural PVC films can be recycled and reused after refinement.

PVC bottles in hard products are also a widely used product, and their waste can be processed into molded materials, pipes and folding plates. In plastic processing plants, PVC fragments are first cleaned, bottle caps and labels are removed, dried, and then crushed into small pieces smaller than 0.5mm. BFGoodrich, one of the major PVC manufacturers in the United States, conducted a test using new PVC and recycled PVC for co-extrusion. The results showed that recycled PVC preserved the excellent properties of new materials and can be used in situations with lower performance requirements.

For physical recycling methods, the quality of PVC recycled materials has a greater relationship with the degree of contamination and the composition of the selected materials. The quality of the recyclate determines the extent to which the original material can be replaced by recycled material: "high-quality" recyclate can be reused in the same type of application as PVC, whereas "low-quality" recyclate from the mixed waste fraction can only be recycled. "Low-level recycling" into products typically made from other materials.

Of course, the quality of the final product is also inseparable from the recycling equipment itself. At present, Germany's equipment and processes for recycling and processing PVC waste have reached a very high level. The contaminant content is less than 1%, and the obtained PVC secondary crushed material can be processed into many products using various methods. The Polyvinyl Chloride Association of the American Plastics Industry Association also strongly encourages the recycling of PVC and plans to start with the recycling of PVC bottles and promote the recycling of other PVC products.

chemical recycling method

Chemical recycling refers to using the chemical properties of polymers to convert polymers into small molecular compounds or simple compounds. The chemical decomposition method has a high utilization rate of resources, but the process is complicated and the cost is high, which affects the recycling of plastic products.

For PVC, in addition to the fragmentation of the polymer chain, the chlorine atoms connected to the chain are also released in the form of HCI. HCI is either reused after purification or must be neutralized to form different products, some of which can be reused and some of which must be disposed of, depending on the technology used in the treatment process. Countries around the world have stepped up research in this field.

The waste plastic recycling technology developed in China can recycle waste PVC mulch films and shed films to produce oil products, paraffin, building materials, etc. Under the condition of continuous production using this technical equipment, the daily processing capacity of waste agricultural film is strong, the oil yield rate can reach 40% to 80%, the conversion rate of gasoline and diesel is high, and it meets the standards of vehicle fuel and environmental emission standards. In addition, since PVC film contains 30% to 37% dioctyl phthalate, room temperature solvent extraction DOP technology can be used to recover the solvent at a temperature below 800°C. DOP is a widely used plasticizer. Recycling 1 ton of waste PVC film can make a profit of more than 2,000 yuan and eliminate environmental pollution.

Europe also has two more advanced solvent-based PVC recycling processes. The Belgian company Solvay invested 10.6 million euros to build the world's first soft PVC resin recycling device in Italy. The device uses the VinyLoop batch process to regenerate 8,500 tons of PVC from 10,000 tons of waste plastic (mostly composed of PVC) every year. The technology uses a mixture of methyl ethyl ketone as the solvent, and selectively dissolves PVC and additives under pressurized conditions at 100°C to 140°C. The solution can be precipitated to obtain soft PVC containing additives. The cost of recycling PVC through this process is equivalent to producing new PVC, and the resulting PVC contains additives, so the resin sells at a higher price. The use of VinyLcop technology can reduce the environmental impact of PVC, especially soft PVC. A second VinyLoop unit was commissioned in France in 2004 and was designed to recycle PVC-coated linoleum and fabrics.

Another new solvent-based PVC recovery process in Europe is used to recover PVC from entire automotive wiring boards. This process was jointly developed by automobile manufacturers in Wuppertal and Delphi, Germany, and Wuppertal University. It was industrialized three years ago by Wiefek GmbH, an automobile parts recycler in Nohfelden-Fisen, Germany. Using this process to recycle PVC, the consumption is nearly 20% lower than the price of new PVC. This process uses esters and ketones as solvents, but the amount of solvent used is much less than that of the Vinyloop process. The solvent does not completely dissolve the waste PVC, but only softens it, making it easy to separate from the copper wire, and the resulting copper wire can be used to pay for the cost of the PVC recycling process. Wiefek has invented and patented a process for separating plastics and solvents using centrifugal methods. The company's PVC recycling production line established based on this process can recycle 225 tons of reusable PVC every year.

heat recovery method

At present, obtaining energy by burning PVC products is also one of the methods of reusing PVC. But this method has a tendency to be eliminated.

During the incineration process, PVC waste produces hydrochloric acid (hydrogen chloride) and is released with the flue gas. These gases need to be neutralized. The method is to add neutralizing substances into the incinerator, the most commonly used one is limestone, so that the acidic components in the incineration gas can be neutralized. Since the chlorine gas released when PVC is burned has strong corrosive and destructive power and is the main cause of dioxins, environmental groups in various countries have strongly opposed the use of incineration to recover heat energy.

The recycling of PVC plastic generally refers to the processing of used PVC through crushing, cleaning, drying, heating and plasticizing, drawing, cooling, granulation, and processing to produce pellets of PVC for re-making into PVC-related products. Especially when used as a raw material, the amount is larger. Since PVC is widely used in pipes and can only be used once due to hygiene and other reasons, equipment for recycling PVC has always been of concern. Generally, PE and PP are separated from PVC, which can also be recycled and granulated in a centralized manner (using DY waste plastic recycling granulator to make PP, PE, ABS, etc.). The recycling of various waste plastics has attracted widespread attention. Generally speaking, plastic crushers are universal, and other equipment are the same as PVC recycling equipment. The equipment configurations for recycling are different, and the quality of the recycled plastics is also different. The key is cleaning and manufacturing. The quality of the grain.

The general plastic recycling and processing process is:

Pvc plastic→PVC plastic→automatic cleaning→plasticizing→drawing and cutting→quality inspection packaging