EPS foamed polystyrene
PA polyamide
PET polyethylene terephthalate.
PE is polyethylene .
PVC is polyvinyl chloride.
PP is polypropylene.
ABS is the combination of acrylonitrile, butadiene and styrene. Polymer.
① Polyvinyl chloride (PVC) It is the largest plastic used in construction. The density of rigid polyvinyl chloride is 1.38~1.43g/cm3, with high mechanical strength and good chemical stability ②Polyethylene (PE) ③Polypropylene (PP) The density of polypropylene is the smallest among all plastics, about 0 ．90 or so. Polypropylene is commonly used to produce construction products such as pipes and sanitary ware. ④Polystyrene (PS) Polystyrene is a colorless, transparent, glass-like plastic. ⑤ABS plastic ABS plastic is a modified polystyrene plastic, composed of three components based on acrylonitrile (A), butadiene (B) and styrene (S).
PS: Polystyrene
It is a colorless and transparent plastic material. It has a glass transition temperature higher than 100 degrees Celsius, so it is often used to make various disposable containers that need to withstand the temperature of boiling water, as well as disposable foam lunch boxes.
PP: Polypropylene
It is a semi-crystalline thermoplastic. It has high impact resistance, strong mechanical properties, and is resistant 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 from polypropylene.
PE: Polyethylene
It is one of the most commonly used polymer materials in daily life and is widely used in the manufacture of plastic bags, plastic films, and milk buckets.
Polyethylene is resistant to a variety of organic solvents and acid and alkali corrosion, but it is not resistant to oxidizing acids, such as nitric acid. Polyethylene will be oxidized in an oxidizing environment.
Polyethylene can be considered transparent in its film state, but when it exists in bulk, it will be opaque due to strong light scattering due to the presence of a large number of crystals inside it. The degree of crystallization of polyethylene is affected by the number of branches. The more branches, the more difficult it is to crystallize. The crystal melting temperature of polyethylene 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 temperatures above 130 degrees Celsius.
Structural formula: - CH2 - CH2 - CH2 - CH2 - CH2 - CH2 - CH2 - CH2
ABS: a synthetic plastic made of acrylonitrile, butadiene, and styrene
The graft polymerization products of three monomers, acrylonitrile, butadiene and styrene, are named after the first letters of their English names. It is a resin with high strength, good toughness and excellent comprehensive properties. It has a wide range of uses and is often used as engineering plastics. Industrially, polybutadiene latex or styrene-butadiene rubber with low styrene content is mostly used as the main chain, and is grafted and polymerized with a mixture of acrylonitrile and styrene monomers. In fact, it is often a mixture of butadiene-containing graft polymer and acrylonitrile-styrene polymer SAN (or AS). In recent years, two monomers, styrene and acrylonitrile, have been polymerized first, and then mixed with graft-polymerized ABS resin in different proportions to produce various ABS resins suitable for different uses. Industrial production began in the United States in the mid-1950s.
Industrial production methods can be divided into two categories: one is to mechanically mix polybutadiene or styrene-butadiene rubber and SAN resin on a roller, or to mix the two latexes together. Mix and then polymerize; the other is to add styrene and acrylonitrile monomers to polybutadiene or styrene-butadiene latex with low styrene content for emulsion graft polymerization, or then combine it with SAN resin to Mix in different proportions.
Structure, properties and applications In ABS resin, rubber particles are in the dispersed phase and dispersed in the continuous phase of SAN resin. When impacted, the cross-linked rubber particles withstand and absorb this energy, dispersing the stress and preventing the development of cracks, thereby improving tear resistance.
The purpose of graft*** polymerization is to improve the compatibility and adhesion between the rubber particle surface and the 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 in acrylonitrile content between the two resins should not be too large, otherwise the compatibility will be poor and cracks will occur at the interface between the rubber and the resin.
ABS resin can be processed into plastic by molding methods such as injection molding, extrusion, vacuum, blow molding and roll pressing. It can also be used for secondary processing by machinery, bonding, coating, vacuum evaporation and other methods. Due to its excellent comprehensive properties and wide range of uses, it is mainly used as engineering materials and can also be used in household appliances. Because of its good resistance to oil, acid, alkali, salt and chemical reagents, and its electroplatability, it has the advantages of good gloss, light specific gravity and low price after being plated with a metal layer, so it can be used to replace certain metals. Many varieties such as self-extinguishing and heat-resistant types can also be synthesized to suit various uses.
PET: Polyethylene terephthalate
Polymer of terephthalic acid and ethylene glycol. The English abbreviation is PET, which is mainly used to make polyethylene terephthalate fiber (the Chinese trade name is polyester). This fiber has high strength and good fabric wearing properties. It is currently the most productive variety of synthetic fibers. In 1980, the world's output was about 5.1 million tons, accounting for 49% of the world's total synthetic fiber output.
Property Molecules The high degree of symmetry of the structure and the rigidity of the p-phenylene chain make this polymer have the characteristics of high crystallinity, high melting temperature and insolubility in general organic solvents. The melting temperature is 257~265℃; its density increases with the crystallinity The density of the amorphous state is 1.33 g/cm^3. Due to the increased crystallinity after stretching, the density of the fiber is 1.38-1.41 g/cm^3. From X-ray studies, the density of the complete crystalline body is calculated. The density is 1.463 g/cm^3. The glass transition temperature of amorphous polymer is 67°C; crystalline polymer is 81°C. The melting heat of the polymer is 113~122 J/g, the specific heat capacity is 1.1~1.4 J/(g.K), the dielectric constant is 3.0~3.8, and the specific resistance is 10^11 10^14 ohm.cm. PET is insoluble in ordinary solvents and is only soluble in some highly corrosive organic solvents such as phenol, o-chlorophenol, m-cresol, and mixed solvents of trifluoroacetic acid. PET fiber is stable to weak acids and weak bases.
Application: Mainly used as raw material for synthetic fibers. Short fibers can be blended with cotton, wool, and linen to make clothing textiles or interior decoration fabrics; filaments can be used as clothing or industrial yarns, such as filter cloths, tire cords, parachutes, conveyor belts, and safety Wait. The film can be used as a base for photosensitive films and audio tapes. Injection molded parts can be used as packaging containers.
PVC: Polyvinyl chloride
It is a polymer material that uses a chlorine atom to replace a hydrogen atom in polyethylene.
The biggest feature of polyvinyl chloride is that it is flame retardant, so it is widely used in fire protection applications. But PVC releases hydrochloric acid and other toxic gases during combustion.
Structural formula: - CH2 - CHCl - CH2 - CHCl - CH2 - CHCl -
POM: polyoxymethylene
The scientific name is polyoxymethylene, which is A thermoplastic crystalline polymer. The English abbreviation is POM. The structural formula is CH —O. Before 1942, most of the products obtained by formaldehyde polymerization were polyoxymethylene glycol HO CH O H, which had a low degree of polymerization and was easily depolymerized by heat. Among them, those with = 8 to 100 were paraformaldehyde; more than 100 is polyoxymethylene. Around 1955, the American DuPont Company polymerized formaldehyde to obtain formaldehyde homopolymer, namely homopolyoxymethylene, with the trade name Delrin. Celanese Corporation of the United States started from trimerformaldehyde and produced a polymer with a small amount of dioxane or ethylene oxide, that is, polyformaldehyde, with the trade name Celcon.
Properties Polyoxymethylene is easy to crystallize, with a crystallinity of 70%; the crystallinity can be increased by high-temperature annealing. The melting temperature of homopolymer is 181°C and the density is 1.425 g/cm. ***The melting point of polyoxymethylene is around 170°C. The glass transition temperature of homopolymer is -60°C. Phenolic compounds are the best solvents for polyoxymethylene. From the study of melt index, it is known that homopolymer has a narrow molecular weight distribution. In addition to strong acids, oxidants and phenol, polyoxymethylene is very stable to other chemical reagents, while homopolymer is also unstable to concentrated ammonia. Stabilized polyoxymethylene can be heated to 230°C without significant decomposition.
Polyformaldehyde can be formed by compression, injection, extrusion, blow molding and other methods. The processing temperature is 170~200℃; it can also be processed by machine tools and welded. The product is light, hard, rigid and elastic, has stable dimensions, small friction coefficient, low water absorption, good insulation properties, and is resistant to organic solvents; it can be used in a wide temperature range (-50~105℃) and humidity range; In various
Technical Department; Feng Zhenxing
April 2, 2010
Understanding of plastics
ABS is the company’s most A common kind of plastic. The natural color of the raw material of this plastic is beige; the most common products include the front frame of the TV; the corners for fixing the four corners of the picture tube, etc.; the characteristics of this plastic are flexibility; very good rigidity; this material has the largest The special feature is that it can be electroplated.
The first identification method; the naked eye identification method mainly depends on whether the inside of the product is beige. If so, it can be concluded that it is ABS material. The appearance of ABS material is relatively smooth
The second identification method; The physical test method can identify ABS in similar colors and products. It is harder and more flexible than other similar materials. It may not be ABS material. Why is it possible that it is particularly fragile? Because some products are older and have undergone a long period of time. The weathering of time can reduce the organic matter of the product; this method is difficult to judge for products that are more than ten years old.
Identification method three; Flame identification method When ABS burns, the smoke is very thick. The flames turn red and the burned part is in a charred state
Waste plastic recycling is a systematic project. To distinguish between various types of waste plastics, I’m afraid you have to buy some books on polymer materials! Ya Zhijiang makes some brief introduction here to see if it is helpful to you. There are many specific classifications of plastics. As far as polymer materials are concerned, it may not be covered in a few words. But as far as plastics are concerned, there are several classification methods: thermosetting plastics and thermoplastic plastics. The definition of thermosetting plastics: polymer resin is heated by heating Substances that are plasticized or plasticized by the introduction of additives and cannot be thermoformed again after cooling, solidification, and shaping, such as phenolic plastics, urea-formaldehyde plastics, 191 resin tempered plastics, etc. That is, thermosetting plastics cannot be recycled and granulated again. Definition of thermoplastic: Polymer resin is plasticized by heating. After cooling and shaping, it can be heated and plasticized again as needed, and the cycle starts again. Plastic recycling and granulation refers to this type of plastic. Further classification of thermoplastics can be divided into conventional thermoplastic general-purpose plastics and engineering plastics. Commonly used thermoplastic general-purpose plastics include polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), etc. Engineering plastics include Acrylonitrile Butadiene Styrene (ABS) High Impact Polystyrene (AS) or (HIPS). The following methods can be used for simple plastic identification: Intuitive identification method refers to using human senses to experience some intuitive characteristics of plastics. Appearance: Transparent? translucent? opaque? What's the color like (when not dyed)? Put it in the water and float? sink? Smell with your nose: Is there any smell? What smell? Touch it with your hands: smooth or rough? Feeling cold or hot? Use your fingernail to scratch it. Are there any marks? Stretch it with your hands. Is it hard or soft? Is it tough and elastic? Drop the plastic and listen to the sound. Is it loud? Crisp? Or depressed? fragile? Or tenacity? These sensory tests can identify the type of plastic. (PE) Polyethylene The raw material of LDPE is white waxy and transparent; HDPE is white powder or white translucent granular resin. Floating in water, odorless and tasteless, with a waxy smooth feel, leaving traces after scratching, and the film is soft and stretchable. LDPE is soft, extensible, bendable, but easy to break; MDPE and HDPE are harder, have better rigidity and toughness, and have a deeper sound
(PP) The raw material of polypropylene is white waxy, translucent, and can be dissolved in water Floating, odorless and tasteless, smooth to the touch, no trace after scratching, bendable, not easy to break, good tensile strength and rigidity, loud sound (PS) Polystyrene standard glass is transparent; impact resistant and matte, can be used under water Heavy, odorless, tasteless, smooth to the touch, brittle, easy to break, it makes a metallic sound when hit with a nail bullet, commonly known as "sound glue"
ABS is milky white or beige, amorphous, opaque, matte, and It is odorless and tasteless when sinking in water. The material is tough, hard and has good rigidity. Not easy to break, the sound is crisp and clear
(PVC) polyvinyl chloride products vary depending on the plasticization and filler conditions, and some are opaque.
Sink in water, depending on the type. Hard products become soft and bendable when heated to 50°C; soft products will sag, and some are elastic. Hard products such as doors, windows, sewer pipes, etc.
PA-6
PA-66 polyamide (nylon) raw material is milky white, like gum. When heated to above 250°C, it becomes a syrup. It sinks in water. It is odorless and tasteless. The surface is hard and has a hot feeling. It will not break when gently hammered. The sound is low.
PMMA polymethyl methacrylate (organic glass) is as transparent as glass and has a beautiful appearance. It sinks in water, is odorless and tasteless, can be bent freely when heated to 120°C, can be processed by hand, is hard, not brittle and has a dull sound when played with fingers
PTEE is white waxy, with low transparency. Smooth, non-flammable, non-absorbent and extremely weather-resistant. It sinks in water, is odorless and tasteless, has a sense of lubrication, and has a low sound
PU has five forms, including foam, elastomer, coating, synthetic leather and adhesive, with different forms. Some sink, some float. Odorless and tasteless, it varies with different forms. The raw material of low-pitched polycarbonate (PC) is white crystalline powder, light yellow to amber in color, transparent solid, and the product is close to colorless. It is an advanced insulating material, odorless and tasteless, has a metallic feel, is hard, has great resistance when bending, is impact-resistant, has strong toughness, and is noisy. The combustion identification method can be to cut a small piece of plastic sample and clamp it with tweezers. Place it on a lit alcohol lamp or lighter to burn, and carefully observe how easy it is to burn, whether it continues to burn after leaving the fire source or whether it is extinguished immediately, the color of the flame, the smoke emission, and what state changes the plastic undergoes during and after burning. What smell does it smell like when it burns? Determine the type of plastic based on its combustion characteristics. Thermoplastic plastics soften, melt, and even coke when burned; thermosetting plastics become brittle and coke, but do not soften when burned. Plastics containing chlorine, phosphorus, fluorine and silicon are not easy to burn and are self-extinguishing. Plastics containing sulfur and nitro are extremely easy to burn. Some plastics emit black smoke when burning, and some plastics decompose and produce special smells when burning. ...These combustion phenomena can be used as the basis for identifying plastics and differentiating varieties.
Plastic name: Difficulty in burning and conditions after leaving the fire. Flame characteristics. Changes in plastic state. Smell.
PE can burn and continue to burn brightly. The bottom is blue and the upper end is yellow. The melt continues to burn after dripping. The smokeless melt drips and the candle blows out the smell.
PP is yellow at the top, blue at the bottom, with a small amount of soft black smoke, and a pungent smell of bubbling petroleum.
PS is flammable and bright, orange-yellow, with thick black smoke that melts and rises. Bubbles, slightly burnt aromatic smell (styrene monomer smell)
ABS yellow flame, bright, black smoke softened, melted, burnt, no dripping with rubber smell
PA (Polyamide) Slow burning, slow extinguishing, yellow-orange, blue melting at the edge, dripping, bubbling like the special smell of burnt wool, nails
PC yellow, bright, sooting, softening, melting, bubbling, The smell of burnt flowers and fruits
PVC is refractory to fire and extinguishes yellow-orange, with green edges and white smoke, and is softened by light green and yellow flames. It can be drawn and has a pungent hydrogen chloride smell
< p>UF self-extinguishing yellow, light blue at the top, swelling, cracking, turning white, coking formaldehyde smell, ammonia smellMF light yellow, white edges swelling, cracking, turning white, coking formaldehyde smell, light Fishy smell
PF glows, yellow spark cracks, darkens phenol and formaldehyde smell
PF (wood powder) slow burning yellow, black smoke expands, cracked wood and phenol smell
PF p>
CP is an amorphous, odorless, colorless, highly transparent colorless or slightly yellow thermoplastic engineering plastic with excellent physical and mechanical properties, especially excellent impact resistance, tensile strength, and flexural strength. , high compressive strength; small creep, dimensional stability; 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 in Long-term use at -60~120°C; no obvious melting point, in molten state at 220-230°C; due to the high rigidity of the molecular chain, the resin melt viscosity is high; the water absorption rate is small, the shrinkage rate is small, the dimensional accuracy is high, and the dimensional stability is good , the film has low air permeability; it is a self-extinguishing material; it is stable to light, but not resistant to ultraviolet light, and has good weather resistance; it is resistant to oil, acid, strong alkali, oxidizing acids, amines, and ketones, and is soluble in chlorinated hydrocarbons And aromatic solvents, long-term exposure in water can easily cause hydrolysis and cracking. The disadvantages are that it is prone to stress cracking due to poor fatigue resistance, poor solvent resistance, and poor wear resistance. PC can be injection molded, extruded, molded, blow molded, thermoformed, printed, bonded, coated and machined. The most important processing method is injection molding. It must be pre-dried before molding, and the moisture content should be less than 0.02%. Trace amounts of moisture will cause the product to produce white turbidity, silver wires and bubbles when processed at high temperatures. PC has considerable forced high elastic deformation ability at room temperature. It has high impact toughness, so it can be processed by cold pressing, cold drawing, cold rolling and other cold forming processes. The molecular weight of PC for extrusion should be greater than 30,000. Gradient compression type screw should be used, the length-to-diameter ratio is 1:18~24, and the compression ratio is 1:2.5. Extrusion blow molding, injection-blow, injection-pull-blow molding can be used. High quality, highly transparent bottle. There are many types of PC alloys to improve the defects of PC such as high melt viscosity (processability) and easy stress cracking of products. PC forms alloys or mixtures with different polymers to improve 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 alloys, etc., have the performance advantages of two materials and reduce costs. For example, among PC/ABS alloys, PC mainly contributes high heat resistance, good toughness and impact strength, high strength and flame retardancy, while ABS can improve Formability, apparent quality, reduced density. The three major application areas of PC are glass assembly industry, automobile industry and electronics and electrical appliance industry, followed by industrial machinery parts, optical discs, packaging, computers and other office equipment, medical and health care, films, leisure and protective equipment, etc. PC can be used as door and window glass, and PC laminates are widely used in protective windows in banks, embassies, detention centers and public places, as well as in aircraft cabin covers, lighting equipment, industrial safety baffles and bulletproof glass.
PC boards can be used to make various signs, such as gasoline pump dials, car dashboards, warehouses and open-air commercial signs, dot sliding indicators. PC resin is used in car camera systems, instrument panel systems and interior decoration systems, and is used as headlight covers. Reinforced car front and rear fenders, reflective mirror frames, door frame covers, operating lever sheaths, spoilers, PCs are used as junction boxes, sockets, plugs and bushings, gaskets, TV conversion devices, and communication under telephone line brackets Cable connectors, switch boxes, telephone switchboards, switchboard components, relay casings, PC can be used as low-load parts, used in household appliance motors, vacuum cleaners, shampooers, coffee machines, toasters, power tool handles, etc. Gears, worm gears, bushings, guides, refrigerator shelves. PC is an ideal material for optical disc storage media. PC bottles (containers) are transparent, light in weight, good in impact resistance, and resistant to certain high temperatures and washing with corrosive solutions. They are recyclable bottles (containers).
Waste plastics are usually disposed of in landfills or incinerated. Incineration will produce a large amount of toxic gases and cause secondary pollution. Landfilling takes up a lot of space; natural degradation of plastic takes more than a hundred years; precipitated additives contaminate soil and groundwater, etc. Therefore, the development trend of waste plastic processing technology is recycling, but the current recovery and recycling rate of waste plastics is low. The reasons include problems in management, policy, and recycling links, but more importantly, recycling technology is not perfect enough.
There are various technologies for recycling waste plastics. There are technologies that can recycle a variety of plastics, and there are also technologies that specifically recycle a single resin. In recent years, plastic recycling technology has made a lot of gratifying progress. This article mainly summarizes the more common technologies.
1 Separation and sorting technology
One of the key links in the recycling of waste plastics is the collection and preprocessing of waste plastics. Especially in my country, an important reason for the low recycling rate is the low level of garbage classification and collection. Since the melting points and softening points of different resins are quite different, in order to better recycle waste plastics, it is best to classify and process a single type of resin. Therefore, separation and screening is an important step in the recycling of waste plastics. For small batches of waste plastics, manual sorting can be used, but manual sorting is inefficient and will increase recycling costs. A variety of separation and sorting methods have been developed abroad.
1.1 Instrument identification and separation technology
The Italian Govoni company first used X-ray detectors and automatic classification systems to separate PVC from mixed plastics [1]. The American Plastic Recycling Technology Research Center has developed an X-ray fluorescence spectrometer that can separate PVC containers from rigid containers with a high degree of automation. The German Refrakt company uses heat source identification technology to separate molten PVC from mixed plastics at a lower temperature by heating [1].
Near-infrared rays have the function of identifying organic materials. Optical filters using near-infrared ray technology [1] can identify plastics at a speed of more than 2,000 times/second. Common plastics (PE, PP, PS, PVC, PET) can be clearly distinguished. When the mixed plastic passes through the near-infrared spectrum analyzer, the device can automatically sort out 5 common plastics at a speed of 20 to 30 pieces/min.
1.4 Flotation separation method
A Japanese materials research institute successfully used common infiltrating agents, such as sodium lignosulfonate, tannic acid, Aerosol OT and saponin, etc. Separate plastic mixtures such as PVC, PC (polycarbonate), POM (polyoxymethylene) and PPE (polyphenylene ether) [4].
1.5 Electric separation technology [5]
Use triboelectricity to separate mixed plastics (such as PAN, PE, PVC and PA, etc.). The principle is that when two different non-conductive materials rub together, they obtain opposite charges through the gain and loss of electrons. The material with a high dielectric constant has a positive charge, and the material with a low dielectric constant has a negative charge. The plastic recycling mixture is in frequent contact in the rotating pot to generate charges, and then is sent to another pot with a charged surface to be separated.
2 Incineration recovery energy
The combustion heat of polyethylene and polystyrene is as high as 46000kJ/kg, exceeding the average value of fuel oil of 44000kJ/kg, and the calorific value of polyvinyl chloride is also Up to 18800 kJ/kg. Waste plastics burn quickly and have low ash content. They are used abroad to replace coal or oil in blast furnace injection or cement rotary kilns. Because PVC combustion will produce hydrogen chloride, which will corrode boilers and pipes, and the exhaust gas contains furans, dioxins, etc.
The United States has developed RDF technology (refuse solid fuel), which mixes waste plastics with waste paper, wood chips, fruit shells, etc., which not only dilutes the chlorine-containing components, but also facilitates storage and transportation. For those waste plastics that are technically impossible to recycle (such as various composite materials or alloy mixed products) and are difficult to regenerate, they can be incinerated to recover heat energy. The advantages are large processing quantity, low cost and high efficiency. The disadvantage is that harmful gases are produced, a special incinerator is required, and equipment investment, loss, maintenance, and operating costs are high.
3 Melt Regeneration Technology
Melt recycling is to heat and melt waste plastics and then re-plasticize them. According to the nature of the raw materials, it can be divided into two types: simple regeneration and compound regeneration. Simple recycling mainly recycles scraps from resin factories and plastic product factories as well as disposable consumer products that are easy to select and clean, such as polyester beverage bottles, food packaging bags, etc. After recycling, its performance is almost the same as that of new materials.
The raw materials for composite regeneration are waste plastics collected from different channels. They have many impurities, complex varieties, diverse forms, and dirt. Therefore, the regeneration processing procedures are relatively complicated, and the separation technology and screening workload are complex. big. Generally speaking, composite recycled plastics are unstable and easily brittle, and are often used to prepare lower-grade products. Such as construction fillers, garbage bags, microporous sandals, raincoats and packaging materials for equipment, etc.
4 Cracking to recover fuel and chemical raw materials
4.1 Thermal cracking and catalytic cracking technology
Due to the continuous deepening of theoretical research on cracking reactions [6-11], domestic Many advances have been made in the development of external cracking technology. Cracking technology is divided into two types depending on the final product: 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 they both convert waste plastics into low molecular substances, the process routes are different. To prepare chemical raw materials, waste plastics are heated in a reaction tower and reach the decomposition temperature (600-900°C) in an ebullating bed. This generally does not produce secondary pollution, but the technical requirements are high and the cost is also high. Cracking and oilification technologies usually include thermal cracking and catalytic cracking.
Japan's Fuji Recycling Company's technology for converting waste plastics into gasoline, kerosene and diesel uses ZSM-5 catalyst to carry out conversion reactions through two reactors to crack plastics into fuel. Each kilogram of plastic can generate 0.5L gasoline, 0.5L kerosene and diesel. US company Amoco has developed a new process that turns 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 catalysts. LPG and aliphatic fuel can be recovered from PE; aliphatic fuel can be recovered from PP, and aromatic fuel can be obtained from PS. Yoshio Uemichi et al. [13] developed a composite catalytic system for the degradation of polyethylene. The catalyst is silica/alumina and HZSM-5 zeolite. Experiments have shown that this catalyst is more effective in selectively producing high-quality gasoline, with a gasoline yield of 58.8% and an octane number of 94.
Domestic Li Mei et al. [14] reported that MON73 gasoline and SP-10 diesel can be obtained from waste plastics at a reaction temperature of 350 to 420°C and a reaction time of 2 to 4 seconds. This process can be continuously produced. Li Wenhong et al. [3] conducted research on catalysts in the waste plastic degradation process. In the catalytic cracking process using PE, PS and PP as raw materials, the ideal catalyst is a molecular sieve catalyst with an acidic surface, an operating temperature of 360°C, a liquid yield of more than 90%, and a gasoline octane number greater than 80. Liu Gongzhao [15] researched and developed a pilot plant for catalytic cracking of waste plastics into gasoline and diesel at one time. It can produce 2 tons of gasoline and diesel per day, and can realize continuous operation of gasoline and diesel separation and slag discharge. The cracking reactor has good heat transfer effect. , characterized by large production capacity. With a catalyst addition amount of 1 to 3% and a reaction temperature of 350 to 380°C, the total yield of gasoline and diesel can reach 70%. The octane numbers of gasoline produced from waste polyethylene, polypropylene and polystyrene are 72 and 77 respectively. and 86, the freezing point of diesel is 3, -11, -22°C. The process is safe to operate and has no three waste emissions. Yuan Xingzhong [16] studied the technology of catalytic cracking of waste plastics in a fluidized moving bed reactor in order to solve the problems of kettle bottom cleaning and pipe cementation. It lays the foundation for achieving safe, stable, long-term continuous production, reducing energy consumption and costs, and improving productivity and product quality.
Cracking waste materials to produce chemical raw materials and fuels is an important way to recover resources and avoid secondary pollution. Germany, the United States, and Japan all have large-scale factories, and my country also has small-scale waste plastic oil chemical plants in Beijing, Xi'an, and Guangzhou. However, there are still many problems to be solved. Due to the poor thermal conductivity of waste plastics, plastics produce high-viscosity melts when heated, which is not conducive to transportation; waste plastics contain PVC, which causes HCl to be produced, which corrodes equipment and reduces catalyst activity; carbon residue adheres to the reactor wall and is difficult to remove, affecting continuous flow. Operation; the service life and activity of the catalyst are low, which makes the production cost high; there is currently no better way to deal with the oil residue generated during production, etc. There are still many domestic reports on pyrolysis to oil [43-54], but how to absorb existing results and overcome technical difficulties is what we urgently need to do.
4.2 Supercritical Oilification Method
The critical temperature of water is 374.3℃, and the critical pressure is 22.05Mpa. Critical water has the properties of an organic solution under normal conditions, can dissolve organic matter but not inorganic matter, and is completely miscible with air, oxygen, nitrogen, carbon dioxide and other gases. Japanese patent report on recycling waste plastics (PE, PP, PS, etc.) with supercritical water. The reaction temperature is 400-600°C, the reaction pressure is 25Mpa, the reaction time is less than 10 minutes, and an oily yield of more than 90% can be obtained. . The advantages of using supercritical water to degrade waste plastics are obvious: water is cheap as a medium; it can avoid carbonization during pyrolysis; the reaction is carried out in a closed system and will not bring new pollution to the environment; the reaction is fast and High production efficiency. Qiu Ting et al. [17] summarized the progress of supercritical technology in waste plastic recycling.
4.3 Gasification technology
The advantage of the gasification method is that it can mix municipal waste without separating plastics, but the operation requires higher temperatures than the thermal decomposition method (usually 900°C about). The Schwaize Pumpe refinery of German company Espag can process 1,700 tons of waste plastic into city gas every year. RWE plans to gasify 220,000 tons of lignite, 100,000 tons of plastic waste and petroleum sludge produced by urban oil processing plants every year. The German company Hoechst uses the high-temperature Winkler process to gasify mixed plastics and then convert them into water gas as a raw material for synthesizing alcohols.
4.4 Hydrocracking Technology
The German Vebaeol Company has established a hydrocracking device to hydrogenolyze waste plastic particles at 15-30Mpa and 470°C to generate a synthetic oil in which the chain Alkanes are 60%, cycloalkanes are 30%, and aromatic hydrocarbons are 1%. The energy efficiency of this processing method is 88%, and the material conversion efficiency is 80%.
5 Other utilization technologies
Waste plastics also have a wide range of uses. Texas State University in the United States uses yellow sand, gravel, liquid PET and curing agent as raw materials to make concrete, and Bitlgosz [18] uses waste plastics as cement raw materials. Xie Liping [19] used waste plastic, wood, paper, etc. to prepare mesoporous activated carbon. Lei Yanying et al. [20] reported the use of waste polystyrene to make coatings. Li Lingling [21] reported that plastic can be turned into wood. Song Wenxiang [22] introduced the use of HDPE as raw material abroad and a special method to produce high-strength plastic sleepers by making glass fibers with different lengths in the mold along the axial direction of the material flow. Pu Tingfang [23] et al. used waste polyethylene to produce 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, so it can well infiltrate the contact surface after melting and play a good bonding role. Zhang Zhengqi et al. [25] used waste plastics to modify asphalt, and evenly dissolved one or more plastics in the asphalt in a certain proportion to improve the road performance of the asphalt, thereby improving the quality of the asphalt pavement and extending the life of the pavement.
Conclusion
The control of white pollution is a huge systematic project, which requires the joint efforts of all departments and industries and the ideological and action commitment of the whole society. Participation and support depend on the improvement of national science and technology awareness and environmental awareness. While formulating regulations and strengthening management, government departments can use the development of environmental protection technology and environmental protection industry as an important channel to stimulate the economy and expand employment, and industrialize the collection, processing and recycling of waste plastics.
At present, my country's recycling and processing enterprises are scattered and small in scale.