The difficult process of plastic recycling

The use of petroleum and coal as raw materials to produce plastic instead of natural polymer materials has undergone a difficult process. An entire generation of outstanding chemists have devoted great efforts to achieving the excellent physical and chemical properties and durability of plastics currently available. Plastics, with its light weight, durability, aesthetics, and low cost, replaced a large number of traditional packaging materials and contributed to a revolution in the packaging industry. However, contrary to the expectations of the people, it is precisely these excellent properties of plastics that produce a lot of durable plastic waste. The large amount of discarded plastic packaging after use has become a major environmental hazard. The main reason is that these plastic wastes are difficult to handle and cannot be decomposed and turned into dust. In the existing municipal solid waste, the proportion of plastics has reached 15%-20%, and most of them are disposable plastic packaging products. The disposal of plastic waste is not only a problem in the plastics industry, but it has now become a widespread concern in the international community of public hazards.

In order to meet the needs of protecting the global environment, the world plastics processing industry has developed many new environmental protection technologies. In respect of saving resources, it is mainly to improve the endurance performance of products, extend the life span, and multi-functionalize the design of the products. In the aspect of resource reuse, it mainly deals with the efficient sorting of plastic waste, separation technology, and high-efficiency melt recycling technology. Chemical recycling technology, completely biodegradable materials, water-soluble materials, and edible films; in terms of reduction technology, the research mainly focuses on the technology of compression and volume reduction of waste plastics and the technology of film-packed containers, and on the premise of ensuring application performance, The thinning technology of products; the development of CFC substitutes is mainly focused on carbon dioxide foaming technology; on the research of substitutes, PVC and PVDC substitutes are mainly developed.

In the field of plastic solid waste disposal in cities, three methods are currently used for landfill, incineration and recycling. Because of different national conditions, different countries, the United States to landfill, Europe, Japan to burn mainly. With landfill treatment, the quality of plastics is generally light, and it is not easy to rot, which will cause the landfill to become a soft foundation and it is difficult to use it in the future. The use of incineration processing, due to the large heat of plastic, easy to damage the stove, plus the gas generated after burning will promote the global warming, and some plastics will also emit harmful gases in the burning and pollute the atmosphere. The use of recycling methods, due to man-hours, high recycling costs, and the lack of appropriate recycling channels, the world's recycling and reuse accounted for only about 15% of the total plastic consumption. However, due to limited world oil resources, from the perspective of saving the earth's resources, the recycling of plastics is of great significance. To this end, all countries in the world have invested a lot of manpower and material resources to develop various key technologies for the recycling of waste plastics, and have devoted themselves to the development of suitable applications for reducing the cost of recycling plastics.

First, the recovery of thermal energy law

Most plastics use petroleum as the raw material, and the main component is hydrocarbons, which can be burned. For example, polystyrene burns more heat than dye oil. Some experts believe that sending plastic waste to an incinerator can provide the heat for heating or power generation. Because 86% of petroleum dyes are directly burned, only 4% of them are made into plastic products, and plastic is used up and sent as heat energy. Burning is normal, and the use of heat energy is one of the last methods for plastic recycling and should not be taken lightly. However, many environmental groups oppose the burning of plastics. They believe that the incineration method will concentrate all the chaos of chemicals and cause toxic gases. If half of the PVC component is chlorine, chlorine emitted from combustion will have a strong erosive effect, and it will be the culprit for causing dioxin.

At present, Germany has 200,000 tons of PVC waste each year, of which 30% is burned in the incinerator, burning people's minds, and the law has to formulate countermeasures. The Federal Environment Agency of Germany has stipulated that all incinerators must meet a limit of less than 0.1 ng (nanograms) per cubic meter of exhaust gas. Although the air pollution standards for incinerators in Germany are already recognized as high standards in the world, they still do not dare to say that the combustion methods will not release harmful substances due to mechanical failure. Therefore, it is foreseeable that environmental protection groups in various countries will strongly oppose the incineration method to recover heat energy.

Second, the classification and recovery method

As plastic recycling, the most important is to classify. Common plastics are polystyrene, polypropylene, low-density polyethylene, high-density polyethylene, polycarbonate, polyvinyl chloride, polyamide, polyurethane, etc. The difference between these plastics is generally difficult to distinguish. Most of the current plastics classification work is done manually. Recent advances in machine classification have led to new research. A German chemical science and technology association has invented infra-red to identify categories. It is fast and accurate, but it has high sorting costs.

Third, the chemical reduction method

Researchers began to try to extract the chemical composition of plastics for reuse. The process used is to cut the long chains of polymers and restore their original properties. The cracked materials can be used to make new plastics. Some methods promote the chemical cracking of bonded carbon atoms by adding chemical elements, or add energy to promote thermal cracking.

German company Bayer developed a hydrolytic chemical reduction method to crack the PUC sponge pad. Tests have shown that the chemical reduction method is technically feasible, but it can only be used to handle clean plastics, such as corner powders and other plastic wastes produced in the manufacturing process. Plastics that have been used in the home and are contaminated with other contaminants are difficult to dispose of using chemical decomposition. The application of this reduction method, until the 21st century will not be a large number of use of hydrolysis to deal with waste. Some new chemical decomposition methods are still in the process of research, the United States Ford Motor Company is currently using esterification method to deal with automotive plastic parts.

The American Institute of Technology in Ransom developed a solution that can break down plastic waste, heating this patented solution together with six different types of plastic that are mixed together. At different temperatures, six polymers can be extracted separately. In the experiment, the polystyrene plastic chips and related solutions were mixed at room temperature into a dissolved state, which was sent to a sealed container for heating, and then sent to a lower pressure “flashing chamber” where the solution quickly evaporated. Recycling, and the rest is pure polystyrene that can be reused.

It was alleged that the purification device used in the research could purify 1 kg of polymer per hour. New York State Government and Niagara. Mohawk Power is planning to join forces to build a small pilot plant. Investors claim that after the plant is completed, 4 tons of polymer material can be recovered per hour. Its cost is only 30% of the raw materials for production and it has very obvious commercial value.

IV. Hydrolysis method

Many experts believe that hydrogenation can be used to process mixed plastic products. The mixed plastic chips are placed in a hydrogen reactor and subjected to a specific temperature and pressure to produce raw materials such as synthetic crude oil and gas. This treatment can be used to treat PVC waste, which has the advantage of not producing toxic dioxins and chlorine. Using this method to process mixed plastic articles, according to different plastic components, 60%-80% of them can be refined into synthetic crude oil. Three chemical companies in Germany and other countries, BASF, pointed out in their common research report that hydrogenation is the best method for thermal cracking, and the synthetic crude oil obtained from the analysis has good quality and can be used for oil refining.

The University of Kentucky in Lexington, United States, invented a process for converting waste plastic into high-quality plastic fuel oil. The fuel produced in this way is very much like crude oil, even lighter than crude oil, and more easily extracted into high-octane fuel oil. This kind of fuel oil produced from waste plastics does not contain sulfur and has very few impurities. A similar method is used to liquefy the plastic with coal. Can also produce high quality fuel oil.

Researchers mixed various plastics with zeolite catalysts, tetralin, etc. in a shower and put it into a reactor called a “pipe bomb”. They pressurize and heat with hydrogen to promote macromolecule plastics. Decomposes into smaller molecular weight compounds. This process is similar to that in crude oil processing. After this waste plastic processing, the oil production rate is very high, and the polyethylene plastic bottle's oil output rate can reach 88%. When waste plastic and coal are mixed and liquefied in a roughly 1:1 ratio, better quality fuel oil can be obtained. After the economic benefits of this process are evaluated, it is expected that the use of waste plastics to produce fuel oil will be transformed into a blast furnace benefit within 5-10 years. At present, Germany has begun to establish a reactor with the hope of producing 200 tons of plastic fuel in Botepo.

Fifth, subtraction design method

Research and development departments taking into account the need for recycling and dismantling when designing products, and materials that are suitable for recycling in the United States, do not focus on which type of plastic should be used to make individual parts, but consider the materials that can be widely used. This is a revolutionary change in conception.

In order to facilitate recycling, designers began to avoid using multiple plastics when designing products. The US company BMW plans to reduce its plastics category by 40% in its new vehicle design, with the aim of facilitating the recycling of waste plastics. The reason why the automotive industry has reduced the use of plastics and considered the additional nature in the design is mainly to expect to win an excellent image that respects environmental protection and is appreciated by consumers. At present, this design concept is gradually infecting the entire plastics processing industry.

However, every effort has not been made to eliminate any of the 20 types of plastics in the market. After all, the diversity of the products has led to the ever-changing categories of plastics, such as the plastics used in the production of computers and the plastics used in the production of automobiles.

To this end, experts recommend the establishment of relevant recycling standards, which stipulate that special industries can only use specified materials, otherwise they cannot control effective recycling. The electronics and automotive industries have already started to establish such standards.

The world's electrical and electronic market has paid attention to the recycling of waste plastics. International Business Machines Corporation (IBM) has begun to code plastic parts for computers and business machines, and is developing reusable plastic electronic components and products that simplify the disassembly of equipment. The structure, while also considering eliminating the surface coloration of components, controls the use of plastic additives to reduce the amount of external adhesive used without the use of recycled process components and additional components.

The recycling of discarded auto parts has also made great progress. Many countries are based on the premise that recyclable and easy recycling can be used as raw material for automotive plastic parts selection and product design. Some countries have already formulated effective recycling numbers and recycling plans for plastic parts for automobiles, and are considering the development of a unified marking system that will help dismantle and sort automotive plastics. Countries such as Europe and the United States are still studying chemical depolymerization to recover automotive plastics.

Six, biodegradation

While actively developing technologies for recycling plastics, research and development of biodegradation has become a research hotspot in the plastic processing industry in various countries in the world. The researchers hope to develop a plastic that can degrade in a microbial environment to deal with the pollution of farmland, forests, and oceans by a large number of disposable plastics, especially plastic film and multi-pack waste. The research goal is to develop a plastic that can guarantee its use performance during use, and once it is used up, it can be decomposed by the microorganisms in the environment and it can completely enter the ecological cycle. At the same time, the production cost of this plastic is relatively low and it has a corresponding economy. If it is such a biodegradable plastic, it can be composted with ordinary biological waste after use, without having to pay a great deal of money for collection, classification and regeneration. Moreover, the decomposition products enter the ecological cycle and no waste of resources occurs.

In the research and development of biodegradable plastics, all countries in the world have invested a lot of financial resources and manpower, and spent a lot of energy on research. The plastics processing industry generally believes that biodegradable plastics are the new technological issues in the 21st century.

At the end of the 1980s, in order to solve the problem of degradation of garbage bags, under the impetus of American corn merchants, polyethylene bags with added starch were popularized as biodegradable plastics in Europe and America. However, since the polyethylene cannot be degraded, its application research has been