Plastic Waste Recycling

Plastic Waste Recycling

1. Research in field of plastic waste recycling technology

Since advent of plastic products in 20th century, they have characteristics of light weight, high strength, corrosion resistance, good chemical resistance, convenient processing, beauty and practicability, and are widely used in various fields around world. However, it is difficult and natural to decompose. Although traditional landfill technology requires less investment and is easy to operate, it will take up a large area of ​​land and cause pollution. Although incineration technology can meet requirements for reduction and at same time recover part of energy, this process easily releases large amounts of hydrocarbons, nitrogen compounds, sulfide compounds and highly toxic dioxins, which directly threaten human health. and ecological environment. Therefore, it is imperative to implement recycling of waste plastics.

In order to increase plasticity and strength of plastics, as well as to provide various product characteristics, a certain amount of additives is added to almost all plastic products. For example, in polyvinyl chloride, a phthalate plasticizer additive is used in an amount of 35% to 50%, over time plasticizer can migrate out of plastic into environment. Plasticizers based on phthalic acid esters have general toxicity and specific toxicity (such as teratogenicity, mutagenicity or activity), play an androgen-like role in humans and animals, disrupt endocrine function, and can cause human reproductive function. Often, number of spermatozoa in men decreases. Moreover, rate of its hydrolysis and photolysis is very slow, and it is a difficult-to-susceptible organic pollutant that remains in air, soil and water. Plastic waste dumped into oceans and rivers around world every year kills a huge number of marine life.

Plastic Waste Recycling

In addition, plastic waste also harms soil and environment. Plastic waste is a refractory polymer compound that is difficult to decompose under natural conditions, when mixed with soil, it destroys good physical and chemical properties of soil, prevents uniform distribution of fertilizers, and affects air permeability of soil. , and does not promote growth of plant roots and affects absorption of substances, nutrients and water, which leads to a decrease in crop yields.

Second, processing

1. Physical treatment:

  • A simple regeneration technique
  • A simple recycling technology is to sort, clean, grind, melt and pellet recycled plastic waste to produce recycled products. It is also used to recycle single-use waste that is easy to clean and sort. Due to simplicity of process, low cost and low investment, simple regeneration technology is widely used.

    However, due to different mixing ratio and compatibility of different plastics, recycled products produced by simple regeneration method have unstable quality, poor performance, become brittle easily, and are not suitable for making high-quality plastic. products, use of which is associated with certain restrictions.

    Plastic Waste Recycling

    ② Physical modification regeneration technology

    Physical modification is addition of various modifiers according to characteristics of different plastic wastes to transform them into useful high value-added materials.

    (1) Filling modification Improving performance of waste plastics by adding fillers, increasing product shrinkage, improving heat resistance, etc.

    (2) Reinforcement modification Add glass fiber, synthetic fiber, natural fiber, etc. to improve strength and modulus of thermoplastic waste, thereby expanding application area.

    (3) Strength modification Use elastomers or blended thermoplastic elastomers to mix with recycled plastic waste for strength modification.

    (4) Mixing Modification Mixing plastic waste with other substances by special means and processing methods, so that modified mixed material has both properties. Reflecting complementary properties of each other, it is also called "polymer alloy".

    ③ Chemical Modification

    Chemical modification refers to introduction of other chain units and functional groups into molecular chain by grafting, copolymerization, etc., cross-linking with cross-linking agents, or treating plastic waste with nucleating agents and blowing agents. , modified treatment.

    (1) Modification by chlorination By modification by chlorination, good fire-resistance and oil-resistance properties can be obtained, which allows product to be used in a wide range of values.

    (2) Crosslinking Modification Through crosslinking, its tensile properties, heat resistance, environmental resistance, dimensional stability, wear resistance, chemical resistance, etc. can be greatly improved. There are three types of crosslinking: radiation crosslinking, chemical crosslinking, and silicone crosslinking.

    (3) Graft copolymerization The purpose of graft modification is to improve adhesion or compatibility of plastics and metals, polar plastics and inorganic fillers. The properties of modified plastics depend on content of grafts, length of grafted chain, etc.

    2. Chemical treatment

  • Thermal oil decomposition technology
  • When heated or heated with addition of a certain amount of catalytic power, plastic is decomposed into original monomers or reduced to oil-like substances, and then chemical raw materials (such as ethylene, styrene, resin, etc.) and liquids are produced Fuels (such as varnish , diesel fuel, liquefied petroleum gas). These are mainly thermal cracking, pyrolysis catalytic cracking and catalytic cracking.

    Pyrolysis is usually heating of plastic waste, which cannot be sorted and contaminated, to ttemperature of their decomposition (600~900°C) in reactor for their decomposition, and then obtain a usable decomposition by absorption and purification.

    All types of waste plastics have their own temperature characteristics of thermal cracking. For general waste plastics such as polyvinyl chloride, polyethylene, polypropylene and polystyrene, staged thermal cracking is usually carried out, such as thermal cracking of polystyrene in low temperature stage, and high value styrene monomer and light fuel oil can be recovered, and fuel oil is recovered in high temperature stage.

    Due to high temperature of pyrolysis reaction, it is difficult to control it, and requirements for equipment materials are also high. Catalytic cracking is often used to reduce reaction temperature and operating costs, as well as increase yield.

    The oil-to-oil pyrolysis technology has many advantages: less nitrogen oxides and sulfur oxides are formed, less amount of spoiled organic substances in pyrolysis residue. However, this method also has some problems, such as high catalyst price, short service life, large investment in equipment, complex process, difficult operation, and cannot be mass-produced. It must be combined with collection, sorting, pre-treatment and etc. In order to implement industrial application of technologies for rectification, purification and other hydrocarbons.

  • Supercritical water and oil technology
  • Supercritical water-to-oil technology uses supercritical water as a medium to decompose plastic waste quickly and efficiently. Because this method has advantages of fast decomposition rate, less secondary pollution, and relative economy, it has become center of research at home and abroad.

    Degradation studies of PS and PS/PP blended plastics in supercritical water show that PS can fully degrade within 1 hour at 380°C; PS/PP with a mass ratio of 7/3 can decompose at a temperature of 390°C. Under these conditions, it can completely decompose within 1 hour.

    The advantages of supercritical water degreasing technology: decomposition reaction speed is fast, carbonization phenomenon occurring during thermal decomposition can be avoided, and reaction does not pollute environment. But at same time, there are also following problems: it must be carried out in a high temperature and high pressure environment, equipment investment is large, and operating costs are difficult to reduce.

  • Heat energy technology
  • Waste and old plastics are mostly carbon and hydrogen. The chemical composition is close to fuel oil, and calorific value reaches 33.6~42 MJ/kg. The technology of utilization of thermal energy consists in processing of heat. energy wastes plastics that are difficult to recycle by incineration.

  • Confocal technology
  • The waste plastic and coal co-coking technology is a newly developed industrial and practical technology that can process mixed plastic waste on a large scale. It is based on technology of high-temperature dry distillation of existing coke oven. Waste plastics are mixed with coking coal in a certain proportion. including tar and diesel fuel used as chemical feedstock) and 40% coke oven gas (used for power generation, etc.). The products are processed according to conventional coke oven product processing method in coking process, in order to realize use of resources and harmless recycling of plastic waste. This process is based on coke oven, tar regeneration system, gas purification and recirculation system of existing metallurgical enterprises. It does not require modification of traditional coking process. It can be put into production and applied only by adding crushing, mixing and molding equipment, which greatly reduces initial investment and operating costs of traditional plastic pyrolysis process.

    The advantages of using plastic waste and coal coking technology: need for plastic waste raw materials is relatively low, process is simple, and equipment investment is low. The plastic waste recycling process is completely closed, and plastic waste is not directly incinerated, which prevents formation of highly toxic substances such as dioxins.

    However, catalyst has a great influence on effect of liquefaction reaction, so it is very important for the compressing system to choose a suitable catalyst, and it is also very difficult.