PA polyamide
PET polyethylene terephthalate.
PE is polyethylene.
PVC is polyvinyl chloride.
PP is polypropylene.
ABS is a polymer of acrylonitrile, butadiene and styrene.
(1) polyvinyl chloride (PVC) is the most used plastic in buildings. The density of rigid PVC is 1.38 ~ 1.43g/cm3, with high mechanical strength and good chemical stability. The density of polyethylene (PE) and polypropylene (PP) is the smallest of all plastics, about 0.90. Polypropylene is usually used to produce building products, such as pipes and sanitary wares. (4) Polystyrene (PS) Polystyrene is a colorless and transparent glassy plastic. ⑤ABS plastic ABS plastic is a kind of modified polystyrene plastic, which consists of acrylonitrile (a), butadiene (b) and styrene (s).
polystyrene
It is a colorless and transparent plastic material. Its glass transition temperature is higher than 100 degrees Celsius, so it is often used to make various disposable containers that need to withstand the temperature of boiling water, namely disposable foam lunch boxes.
Polypropylene: polypropylene
It is a semi-crystalline thermoplastic. High impact resistance, strong mechanical properties, resistance to various organic solvents and acid and alkali corrosion. It is widely used in industry and is one of the common polymer materials. Australian coins are also made of polypropylene.
Polyethylene: polyethylene
It is one of the most commonly used polymer materials in daily life, and is widely used to make plastic bags, plastic films, milk barrels and so on.
Polyethylene is resistant to all kinds of organic solvents, acids and bases, but not to oxidizing acids, such as nitric acid. Polyethylene will be oxidized in an oxidizing environment.
Polyethylene can be regarded as transparent in the film state, but when it exists as a block, it will be opaque because of strong light scattering because there are a lot of crystals in it. The crystallinity of polyethylene is affected by the number of branches. The more branches there are, the more difficult it is to crystallize. The melting temperature of polyethylene crystals is also affected by the number of branches, ranging from 90 degrees Celsius to 130 degrees Celsius. The more branches, the lower the melting temperature. Polyethylene single crystals can usually be prepared by dissolving high-density polyethylene in xylene at an environment higher than 130 degrees Celsius.
Structural formula:-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2.
ABS: It is a synthetic plastic of acrylonitrile, butadiene and styrene.
Graft polymerization products of acrylonitrile, butadiene and styrene are named after their initials. It is a kind of resin with high strength, good toughness and excellent comprehensive performance, which is widely used and often used as engineering plastics. In industry, polybutadiene latex or styrene-butadiene rubber with low styrene content is the main chain, and it is obtained by graft polymerization with a mixture of acrylonitrile and styrene. In fact, it is often a mixture of graft polymer containing butadiene and acrylonitrile-styrene polymer SAN (or AS). In recent years, styrene and acrylonitrile were polymerized first, and then mixed with ABS resin grafted with styrene and acrylonitrile in different proportions to prepare various ABS resins suitable for different purposes. In the mid-1950s, the United States began industrialized production.
Industrial production methods can be divided into two categories: one is to mechanically mix polybutadiene or styrene-butadiene rubber with SAN resin on a roller, or to polymerize after mixing two kinds of latex; The other is that styrene and acrylonitrile monomers are added to polybutadiene or styrene-butadiene latex with low styrene content for emulsion graft copolymerization, or mixed with SAN resin in different proportions.
Structure, properties and application in ABS resin, rubber particles are dispersed in the continuous phase of SAN resin. When impacted, the crosslinked rubber particles bear and absorb this energy, which disperses the stress, thus preventing the crack from developing and improving the tear resistance.
The purpose of graft polymerization is to improve the compatibility and adhesion between rubber particle surface and resin phase. This is related to the amount of free SAN resin and the composition of SAN resin grafted on the rubber backbone. The difference of acrylonitrile content between the two resins should not be too big, otherwise the compatibility is not good, which will lead to the interface cracking between rubber and resin.
ABS resin can be processed into plastic by injection molding, extrusion, vacuum, blow molding, calendering and other methods, and can also be processed by mechanical, adhesive, coating, vacuum steaming and other methods. Because of its excellent comprehensive performance and wide application, it is mainly used as engineering materials and household appliances. Because of its good oil resistance, acid resistance, alkali resistance, salt resistance and chemical reagent resistance, as well as its electroplating property, the plated metal layer has the advantages of good luster, light specific gravity and low price, and can be used to replace some metals. It can also be synthesized into self-extinguishing, heat-resistant and other varieties to adapt to various uses.
PET: polyethylene terephthalate
Terephthalic acid, a polymer containing ethylene glycol, is abbreviated as PET, which is mainly used to make polyethylene terephthalate fiber (the trade name of which is polyester in China). This kind of fiber has high strength and good wearability. At present, it is one of the most productive synthetic fibers. 1980 The world output was about 565,438+ten thousand tons, accounting for 49% of the world's total synthetic fiber output.
The high symmetry of molecular structure and the rigidity of p-phenylene chain make the polymer have the characteristics of high crystallinity, high melting temperature and insolubility in general organic solvents, and the melting temperature is 257 ~ 265℃. Its density increases with the increase of crystallinity. The density of amorphous fiber is 65438 0.33 g/cm3. After stretching, due to the increase of crystallinity, the fiber density is1.38 ~1.41g/cm3. According to the X-ray study, the density of the complete crystal is 65438+. The glass transition temperature of amorphous polymer is 67℃; The crystalline polymer is 865438 0℃. The heat of fusion of the polymer is113 ~122 j/g, the specific heat capacity is1~1.4j/(g/k), and the dielectric constant is 3.0 ~ 3.8. PET is insoluble in common solvents, but only soluble in some highly corrosive organic solvents, such as mixed solvents of phenol, o-chlorophenol, m-cresol and trifluoroacetic acid. PET fiber is stable to weak acids and bases.
Uses are mainly used as raw materials for synthetic fibers. Short fibers can be blended with cotton, wool and hemp to make textiles for clothing or cloth for indoor decoration; Filament can be used as clothing yarn or industrial yarn, such as filter cloth, tire cord, parachute, conveyor belt, safety belt and so on. Film can be used as the base of photosensitive film and recording tape. Injection molded parts can be used as packaging containers.
PVC: polyvinyl chloride
It is a polymer material, which uses chlorine atoms to replace hydrogen atoms in polyethylene.
The biggest feature of PVC is flame retardant, so it is widely used in fire protection applications. However, PVC will release toxic gases such as hydrochloric acid during combustion.
Structural formula: -CH2-CHCl-CH2-CHCl-CH2-CHCl-
polyformaldehyde
The scientific name is polyoxymethylene, which is a thermoplastic crystalline polymer. The English abbreviation is POM. The structural formula is CH-O. Before 1942, the products obtained by formaldehyde polymerization were mostly polyoxymethylene ethylene glycol Hoch-o-h, with low degree of polymerization and easy depolymerization when heated, in which paraformaldehyde with = 8 ~100; More than 100 is-polyformaldehyde, about 1955. DuPont Company of the United States polymerizes formaldehyde to obtain formaldehyde homopolymer, that is, homopolyformaldehyde, with the trade name Delrin. Celanese Company of the United States prepared a * * * polymer containing a small amount of dioxolane or ethylene oxide, namely * * * polyoxymethylene, with the trade name Celcon.
Polyformaldehyde is easy to crystallize, and the crystallinity reaches 70%. By high temperature annealing, the crystallinity can be increased. The melting temperature of paraformaldehyde is 18 1℃, and the density is 1.425 g/cm. * * * The melting point of polyoxymethylene is about 170℃. The glass transition temperature of paraformaldehyde is -60℃. Phenolic compounds are the best solvents for polyoxymethylene. According to the study of melting index, the molecular weight distribution of paraformaldehyde is narrow. Paraformaldehyde is stable to other chemical reagents except strong acid, oxidant and phenol, while paraformaldehyde is unstable to concentrated ammonia water. Stable polyoxymethylene can be heated to 230℃ without obvious decomposition. Polyformaldehyde can be molded by compression, injection, extrusion and blow molding, and the processing temperature is 170 ~ 200℃. It can also be machined and welded by machine tools. Light weight, good hardness, rigidity and elasticity, stable size, low friction coefficient, low water absorption, good insulation performance and resistance to organic solvents; It can be used in a wide temperature range (-50 ~ 105℃) and humidity range; various
Technology department; Feng Zhenxing
April 2, 2065 438+00
Understand plastics
ABS is the most common plastic in our company. The natural color of this plastic is beige. The most common product is the front frame of TV; Fix the angle of the picture tube, etc.'; This kind of plastic is characterized by flexibility; Very good rigidity; The biggest feature of this material is that it can be electroplated.
The identification method is the first; The naked eye identification method mainly depends on whether the interior of the product is beige. If it is, it can be concluded that it is ABS material, and the appearance of ABS material is relatively smooth.
Identification method 2; Physical testing method is more difficult and flexible than other similar materials in identifying ABS with similar color and product. The possibility of being particularly brittle does not mean that ABS materials are possible. Because some products can reduce the organic matter of products after a long period of weathering; This method is not easy to judge in products over ten years old.
Identification method 3; Flame identification method ABS has thick smoke when burning, and the burning part is burnt when the flame is red.
Plastic recycling is a systematic project. I'm afraid I have to buy some books on polymer materials to distinguish all kinds of waste plastics! Yazhijiang will make some brief introductions here to see if it will help you. There are many specific classifications of plastics. As far as polymer materials are concerned, I'm afraid it can't be covered in a few words, but as far as plastics are concerned, they can be divided into the following categories: thermosetting plastics and thermoplastic thermosetting plastics: polymer resins are plasticized by heating or introducing additives, but they can't be molded by thermoplastic plastics after cooling, such as phenolic plastics, urea-formaldehyde plastics, 19 1 resin toughened plastics, etc. In other words, thermosetting plastics cannot be recycled and re-granulated. Definition of thermoplastic: Polymer resin is plasticized by heating, and can be reheated and plasticized as needed after cooling, and so on. Plastic recycling granulation refers to this kind of plastic. Thermoplastics can be further classified into conventional thermoplastics and engineering plastics. Commonly used thermoplastics are polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS) and so on. Engineering plastics include acrylonitrile butadiene styrene (ABS), high impact polystyrene (AS) or (HIPS). Simple plastic identification can be in the following ways: intuitive identification refers to using people's senses to experience some intuitive characteristics of plastic. Visual appearance: transparent? Translucent? Opaque? What color (when not dyed)? Float in the water? A sink? Sniff: Is there any smell? What is that smell? Touch by hand: smooth or rough? Feel cold or hot? Scrape it with your nails. Is there any trace? Stretch by hand. Is it hard or soft? Is it tough and elastic? Put down the plastic and listen to its sound. Is it loud? Crispy or low? Fragile? Still tough? Through these sensory tests, we can identify what kind of plastic it is. (PE) The raw material of polyethylene LDPE is white wax, which is transparent; HDPE is white powder or white translucent granular resin. Floating in water, odorless and tasteless, with waxy smoothness, traces after scraping, and soft and stretchable film. LDPE is soft, extensible and bendable, but it is easy to break. MDPE and HDPE are hard, with good rigidity and toughness and low noise.
(PP) Polypropylene is white, waxy, translucent, floating in water, odorless and tasteless, smooth to the touch, flexible and not easy to break, good tensile strength and rigidity, bright sound (PS) as transparent as polystyrene standard glass; Impact resistance, dull, submerged in water, odorless and tasteless, smooth to the touch, fragile and easily broken. Hit with nails, commonly known as "loop glue."
ABS is milky white or beige, amorphous, opaque and dull, odorless and tasteless when submerged in water, and its material is tough, hard and rigid. Not easy to break, the sound is crisp.
(PVC) PVC products are different due to different plasticization and fillers, and some are opaque. Submerged in the water, depending on the species. Hard products are soft and flexible when heated to 50℃. Soft products will sag, some are elastic, and hard products such as doors and windows, sewer pipes, etc.
PA-6
PA-66 polyamide (nylon) is milky white, such as gum. When heated above 250℃, it becomes water. Submerged in water, odorless and tasteless, the surface is hard and hot, the light hammer will not break, and the sound is low.
PMMA polymethyl methacrylate (PMMA) is as transparent as glass and beautiful in appearance. Submerged in water, odorless and tasteless, can bend freely when heated to 120℃, can be processed by hand, and is hard and not fragile.
PTEE is white wax-like, with low transparency, smoothness, incombustibility, non-absorption and excellent weather resistance. Submerged in water, odorless and tasteless, with a sense of lubrication and a low voice.
PU has five forms: foam, elastomer, paint, synthetic leather and adhesive, which are different. Some are submerged in water and some are floating in water. Odorless and tasteless, depending on the shape. PC raw material is white crystalline powder, light yellow to amber, transparent solid, and the product is almost colorless. It is an advanced insulating material, odorless, metallic, hard, bending-resistant, impact-resistant, tough and loud. The combustion identification method can cut a small piece of plastic sample, clamp it with tweezers and burn it on a lit alcohol lamp or lighter. Carefully observe the difficulty of burning, whether it will continue to burn or go out immediately after leaving the fire source, the color of the flame, the smoke situation, the state change of plastic during and after burning, and the situation during burning. According to the combustion characteristics of plastics, the types of plastics are determined. Thermoplastics will soften, melt and even coke when burning; Thermosetting plastics become brittle and scorched when burning, but they will not soften. Plastics containing chlorine, phosphorus, fluorine and silicon are not easy to burn and have self-extinguishing; Plastics containing sulfur and nitro are very easy to burn; Some plastics emit black smoke when burning; Others will decompose and produce special smells when burning ... These burning phenomena can be used as the basis for identifying plastics and distinguishing varieties. In the name of plastic, it is difficult to burn. After leaving the fire, the characteristics and state of the flame change the smell of plastic.
PE can burn very brightly, the bottom is blue, the yellow droplets at the top lag and continue to burn, and the smokeless candle blows out the smell.
The upper end of PP is yellow, the lower end is blue, a little soft with black smoke, and the taste of foam oil is spicy.
PS is flammable, bright, orange-yellow, with thick black smoke, melting and foaming, and has a slight fragrance (styrene monomer smell).
ABS yellow flame, bright, black smoke softened, melted and burned, without dripping with rubber smell.
PA (polyamide) burns slowly, extinguishing yellow and orange, and the blue edge melts, drips and bubbles like the special smell of burning wool and nails.
The PC is yellow and bright, and smells like softening, melting, foaming and coking flowers and fruits.
PVC is hard to burn, it turns yellow and orange when it leaves the fire, and its edge is green, emitting white smoke, spraying light green yellow flame to soften it, and it can draw the irritating smell of hydrogen chloride.
UF self-extinguishes yellow, and the top light blue swells, cracks and turns white, with the smell of coking formaldehyde and ammonia.
MF is light yellow with white edges, swollen, cracked and turned white, with coking formaldehyde smell and faint fishy smell.
PF glows, the yellow spark breaks, turns black, and smells of phenol and formaldehyde.
PF (wood flour) slowly burns yellow, black smoke expands, wood cracks and smells like phenol.
CP is an amorphous, tasteless, colorless or yellowish thermoplastic engineering plastic with high transparency and excellent physical and mechanical properties, especially excellent impact resistance, high tensile strength, bending strength and compressive strength. Small creep and stable size; It has good heat resistance and low temperature resistance, stable mechanical properties, dimensional stability, electrical properties and flame retardancy in a wide temperature range, and can be used for a long time at -60~ 120℃; It has no obvious melting point and is in a molten state at 220-230℃. Due to the high rigidity of molecular chain, the viscosity of resin melt is high; Small water absorption, small shrinkage, high dimensional accuracy, good dimensional stability and low film permeability; It is a self-extinguishing material; Stable to light, but not resistant to ultraviolet rays and good weather resistance; Oil-resistant, acid-resistant, alkali-resistant, oxidizing acids, amines and ketones, soluble in chlorinated hydrocarbons and aromatic solvents, and easy to cause hydrolysis and cracking in water for a long time. The disadvantages are poor fatigue strength, poor solvent resistance and poor wear resistance, which are easy to cause stress cracking. PC can be injection molded, extruded, molded, blow molded, thermoformed, printed, bonded, coated and machined, and the most important processing method is injection molding. Pre-drying must be carried out before molding, and the moisture content should be less than 0.02%. After a little moisture is treated at high temperature, the product will produce white, silver lines and bubbles. PC has considerable ability to force high elastic deformation at room temperature. It has high impact toughness and can be cold formed by cold pressing, cold drawing and cold rolling. The molecular weight of PC used for extrusion should be greater than 30,000. Step-by-step compression screws should be used, with the length-diameter ratio of 1: 18 ~ 24 and the compression ratio of 1:2.5. Extrusion blow molding, injection blow molding and injection stretch blowing can be used to form bottles with high quality and transparency. There are many kinds of PC alloys, which can improve the defects of high viscosity (processability) of PC melt and easy stress cracking of products. PC and different polymers form alloys or * * * mixtures to improve the material properties. Specifically, there are PC/ABS alloy, PC/ASA alloy, PC/PBT alloy, PC/PET alloy, PC/PET/ elastomer * * * mixture, PC/MBS * * mixture, PC/PTFE alloy, PC/PA alloy and so on. It has the advantages of both materials and reduces the cost. For example, in PC/ABS alloy, PC mainly contributes a lot. The three major application fields of PC are glass assembly industry, automobile industry and electronic appliance industry, followed by industrial machinery parts, compact discs, office equipment such as packaging and computers, medical care, films, leisure and protective articles, etc. PC can be used as door and window glass, and PC laminates are widely used as protective windows in banks, embassies, detention centers and public places, as well as aircraft cabin covers, lighting equipment, industrial safety baffles and bulletproof glass. The PC board can be used as various signs, such as gasoline pump dial, automobile dashboard, warehouse and outdoor commercial signs, point slip indicator, PC resin for automobile camera system, dashboard system and interior decoration system, front lampshade, front and rear fender of automobile, mirror frame, door frame cover, joystick cover, spoiler and PC for communication under junction box, socket, plug and sleeve, gasket, TV conversion device and telephone line bracket. Switch box, telephone switchboard, switchboard assembly, relay shell and PC can be used as low-load parts, which are used in household appliances motors, vacuum cleaners, shampoo machines, coffee machines, toasters, handles of power tools, various gears, worm gears, bushings, guide rails, shelves in refrigerators, etc. PC is an ideal material for optical disk storage media. PC bottles (containers) are transparent, light in weight, good in impact resistance, resistant to certain high temperature and corrosive solution washing, and can be used as recyclable bottles (containers).
Waste plastics are usually disposed of by landfill or incineration. Incineration will produce a large number of toxic gases, causing secondary pollution. Landfill will take up a lot of space; It takes more than one hundred years for plastics to degrade naturally; The precipitated additives pollute the soil and groundwater. Therefore, the development trend of waste plastics treatment technology is recycling, but the recycling rate of waste plastics is low at present. There are problems in management, policy and recycling, but more importantly, the recycling technology is not perfect enough.
There are various technologies for recycling waste plastics, including technologies for recycling various plastics and technologies for recycling single resin. In recent years, plastic recycling technology has made many gratifying progress. This paper mainly summarizes the common technologies.
1 separation and separation technology
One of the key links in plastic recycling is the collection and pretreatment of waste plastics. Especially in China, the important reason for the low recovery rate is the low degree of garbage classification and collection. Because the melting point and softening point of different resins are quite different, in order to recycle waste plastics better, it is best to classify a single variety of resins, so separation and screening is an important link in the utilization of plastic recycling. For small batches of waste plastics, manual sorting can be used, but the efficiency of manual sorting is low, which will increase the recycling cost. A variety of separation and separation methods have been developed abroad.
1. 1 instrument identification and separation technology
The Italian company Govoni used X-ray detector and automatic classification system to separate PVC from mixed plastics for the first time [1]. The American Plastic Recycling Technology Center has developed an X-ray fluorescence spectrometer, which can automatically separate PVC containers from hard containers. Germany's Refrakt Company used heat source identification technology to separate molten PVC from mixed plastics at a lower temperature by heating [1].
Near infrared has the function of identifying organic matter. The optical filter [1] using near infrared technology can identify plastics at a speed of more than 2000 times per second, and ordinary plastics (PE, PP, PS, PVC, PET) can be clearly distinguished. When the mixed plastics pass through the near infrared spectrum analyzer, the device can automatically sort out five common plastics at the speed of 20 ~ 30 pieces per minute.
1.4 flotation separation method
A material research institute in Japan successfully separated PVC, PC (polycarbonate), POM (polyoxymethylene) and PPE (polyphenylene ether) by using common wetting agents, such as sodium lignosulfonate, tannic acid, aerosol OT and saponin [4].
1.5 Electroseparation Technology [5]
SePAration of mixed plastics (such as PAN, PE, PVC and pa) by triboelectricity. The principle is that when two different non-conductive materials rub, they get opposite charges through the gain and loss of electrons, in which the material with high dielectric constant is positively charged and the material with low dielectric constant is negatively charged. Plastic recycling mixture often contacts in a rotating tank to generate charge, and then it is sent to another tank with a charged surface for separation.
Incineration to recover energy.
The combustion heat of polyethylene and polystyrene is as high as 46000 kJ/kg, which exceeds the average value of fuel oil of 44000 kJ/kg, and the calorific value of PVC is as high as18,800 kJ/kg. Waste plastics have fast combustion speed and low ash content, and are used to replace coal or oil in blast furnace injection or cement rotary kiln abroad. Because the combustion of PVC will produce hydrogen chloride, which will corrode boilers and pipelines, and the waste gas contains furan, dioxin and so on. The United States has developed RDF technology (garbage solid fuel), which mixes waste plastics with waste paper, sawdust and fruit shells. It not only dilutes the chlorine-containing components, but also facilitates storage and transportation. For those waste plastics that are technically unrecoverable (such as various composite materials or alloy mixed products) and difficult to regenerate, incineration can be used to recover heat energy. Its advantages are large processing capacity, low cost and high efficiency. Disadvantages are that harmful gases are generated, special incinerators are needed, and equipment investment, loss, maintenance and operation costs are high.
3 melting regeneration technology
Melting regeneration is to re-plasticize waste plastics after heating and melting. According to the properties of raw materials, it can be divided into simple regeneration and compound regeneration. Simple recycling mainly recycles leftover waste from resin factories and plastic products factories, as well as disposable consumer goods such as polyester beverage bottles and food packaging bags, which are easy to select and clean. The performance after recycling is similar to that of new materials.
The raw material of compound regeneration is waste plastics collected from different channels, which has the characteristics of many impurities, complex varieties, diverse forms and dirt, so the regeneration treatment procedure is complicated and the separation technology and screening workload are heavy. Generally speaking, composite recycled plastics are unstable and fragile, and are usually used to prepare lower-grade products. Such as building fillers, garbage bags, microporous sandals, raincoats, equipment packaging materials, etc.
4 cracking to recover fuel and chemical raw materials
4. 1 thermal cracking and catalytic cracking technology
Due to the in-depth study of cracking reaction theory [6- 1 1], the development of cracking technology at home and abroad has made many progress. Cracking technology can be divided into two types due to different final products: one is to recover chemical raw materials (such as ethylene, propylene, styrene, etc.). ) [12], and the other is to obtain fuel (gasoline, diesel, tar, etc.). ). Although all waste plastics are converted into low molecular substances, the process routes are different. The preparation of chemical raw materials is to heat waste plastics in a reaction tower and reach the decomposition temperature (600 ~ 900℃) in a fluidized bed, which generally does not produce secondary pollution, but it has high technical requirements and high cost. Cracking oil technology usually includes thermal cracking and catalytic cracking.
Japan's Fuji Cycle Company converts waste plastics into gasoline, kerosene and diesel oil, and uses ZSM-5 catalyst to crack plastics into fuel through conversion reactions in two reactors. Each kilogram of plastic can produce 0.5L gasoline, 0.5L kerosene and diesel oil. Amoco has developed a new technology to convert waste plastics into basic chemicals in refineries. Pretreated waste plastics are dissolved in hot refined oil and decomposed into light products under the action of high temperature catalytic cracking catalyst. Recovering liquefied petroleum gas and aliphatic fuel from polyethylene; Aliphatic fuel can be recovered from polypropylene and aromatic fuel can be obtained from polystyrene. Yoshio Uemichi et al. [13] developed a composite catalytic system for polyethylene degradation. The catalysts are silica/alumina and HZSM-5 zeolite. The experimental results show that the catalyst can effectively and selectively prepare high-quality gasoline with a gasoline yield of 58.8% and an octane number of 94.
Domestic Li Mei et al. [14] reported that gasoline of MON73 and diesel oil of SP- 10 can be obtained from waste plastics after reacting at 350 ~ 420℃ for 2 ~ 4 s, which can be continuously produced. Li et al. [3] studied the catalysts in the degradation of waste plastics. In the process of catalytic cracking with polyethylene, polystyrene and polypropylene as raw materials, the ideal catalyst is a molecular sieve catalyst with acidic surface, operating temperature of 360℃, liquid yield of over 90% and gasoline octane number of over 80. Liu [15] developed a pilot plant for catalytic cracking of waste plastics to produce steam and diesel oil once, with a daily output of 2 tons, which realized the continuous operation of separation and slag discharge of steam and diesel oil. The cracking reactor has the characteristics of good heat transfer effect and large production capacity. When the amount of catalyst is 1 ~ 3% and the reaction temperature is 350 ~ 380℃, the total yield of gasoline and diesel oil can reach 70%. The octane number of gasoline made of waste polyethylene, polypropylene and polystyrene is 72, 77 and 86 respectively, and the freezing point of diesel oil is 3,-1 1, -22℃. This process Yuan [16] studied the catalytic cracking technology of waste plastics in fluidized moving bed reactor, and solved the problems of slag cleaning at the bottom of the reactor and pipeline cementation. It lays a foundation for realizing safe, stable and long-term continuous production, reducing energy consumption and cost, and improving output and product quality.
It is an important way to recycle resources and avoid secondary pollution to make chemical raw materials and fuels by cracking wastes. Germany, the United States, Japan and other countries have large factories, and China has also built small waste plastic oiling plants in Beijing, Xi, Guangzhou and other places, but there are still many problems to be solved. Due to the poor thermal conductivity of waste plastics, plastics produce high viscosity melt when heated, which is not conducive to transportation; PVC in waste plastics will produce HCl, which will corrode equipment and reduce catalyst activity. Carbon residue adheres to the reactor wall, which is difficult to remove and affects continuous operation; The catalyst has low service life and activity and high production cost; At present, there is no better treatment method for oil residue produced in production, and so on. There are still many reports on pyrolysis to produce oil in China [43-54], but how to absorb the existing achievements and overcome the technical difficulties is an urgent task before us.
4.2 Supercritical refueling method
The critical temperature of water is 374.3℃, and the critical pressure is 22.05Mpa. Critical water has the properties of normal organic solution, which can dissolve organic matter but not inorganic matter, and is completely miscible with air, oxygen, nitrogen and carbon dioxide. Japanese patent reports waste plastics (PE, PP, PS, etc. ) can be recycled with supercritical water. The reaction temperature is 400 ~ 600℃, the reaction pressure is 25Mpa, the reaction time is below 65438±00min, and the oil yield can reach above 90%. The advantages of using supercritical water to degrade waste plastics are obvious: the cost of using water as medium is low; Carbonization can be avoided during pyrolysis; The reaction is carried out in a closed system, which will not bring new pollution to the environment; The reaction speed is fast, and the production efficiency is high. Qiu Ting et al [17] summarized the progress of supercritical technology in plastic recycling.
4.3 gasification technology
The advantage of gasification method is that it can treat municipal solid waste in a mixed way without separating plastics, but the operation needs to be higher than that of thermal decomposition method (generally around 900℃). Schwaize Pumpe Refinery in Espag, Germany can process 1700 tons of waste plastics into city gas every year. RWE plans to gasify 220,000 tons of lignite, more than 654.38 million tons of plastic waste and petroleum sludge produced by town petroleum processing plants every year. Hoechst Company of Germany gasifies the mixed plastics by high-temperature Winkler process, and then converts it into water gas as the raw material for alcohol synthesis.
4.4 Hydrocracking Technology
Vebaeol Company of Germany has set up a hydrocracking unit to hydrogenolysis waste plastic particles at 15 ~ 30 MPa and 470℃ to produce a synthetic oil, which contains 60% paraffin, 30% cycloalkane and 1% aromatic hydrocarbon. The effective energy utilization rate of this processing method is 88%, and the effective substance conversion rate is 80%.
5 Other utilization technologies
Waste plastics also have a wide range of uses. Texas State University uses yellow sand, stones, liquid PET and curing agent as raw materials to make concrete, while Bitlgosz [18] uses waste plastics as cement raw materials. Xie Liping [19] prepared mesoporous activated carbon from waste plastics, wood and paper, Lei et al. [20] reported that waste polystyrene was used as coating, [2 1] reported that plastics could be turned into wood. Song Wenxiang [22] introduced that foreign countries used HDPE as raw material, and through special methods, glass fibers with different lengths were made to flow in the same direction along the axial direction of the material in the mold, thus producing high-strength plastic sleepers. Pu et al. [23] used waste polyethylene to make high value-added polyethylene wax. Li Chunsheng et al. [24] reported that compared with other thermoplastics, polystyrene has the characteristics of low melt viscosity and high fluidity, which can well infiltrate the contact surface after melting and play a good bonding role. Zhang Zhengqi et al. [25] modified asphalt with waste plastics, and dissolved one or several plastics evenly in asphalt according to a certain proportion, which improved the road performance of asphalt, thus improving the quality of asphalt pavement and prolonging the service life of pavement.
Concluding remarks
Controlling white pollution is a huge systematic project, which requires the joint efforts of all departments and industries, the participation and support of the whole society in thought and action, and the improvement of the scientific and technological awareness and environmental awareness of the whole people. While formulating laws and regulations to strengthen management, government departments can take the development of environmental protection technologies and industries as an important channel to stimulate the economy and expand employment, so as to industrialize the collection, treatment and recycling of waste plastics. At present, China's recycling enterprises are scattered and small in scale.