Plastics are difficult to degrade naturally in the environment, and polystyrene is the most. Because of its high molecular weight and high stability, it is generally believed that microorganisms cannot degrade polystyrene plastics. 20 15 authoritative journals in the field of environmental science, such as Professor Yang Jun's research group of Beihang University and Dr. Zhao Jiao of Shenzhen Huada Gene Company &; Technology published two sister research papers, which proved that the larvae of Tenebrio molitor can degrade polystyrene, the most difficult plastic to degrade.
Studies show that Tenebrio molitor larvae can survive for more than 1 month with polystyrene foam as the sole food source, and eventually develop into adults, and the polystyrene they eat is completely degraded and mineralized into CO2 or assimilated into insect body fat. This discovery provides an idea for solving the global plastic pollution problem. Plastic waste pollution in petrochemical production is a worldwide environmental problem. Most plastics are discarded after one-time consumption. Up to now, the academic circles believe that plastic products may not decompose in the natural environment for decades to hundreds of years because of their stable physical and chemical structure.
Professor Yang Jun introduced that in 20 13 years, the global consumption of plastics was 299 million tons, of which polystyrene accounted for 7%, and the annual consumption was about 2 100 billion tons. Common plastic lunch boxes, coffee cups and other materials that can withstand the temperature of boiling water are polystyrene. Authoritative investigation shows that the degradation rate of plastic polystyrene in soil, sludge, rotten garbage or fecal microbial community is only 0.0 1%-3% in four months.
Every year, 40 million tons of waste plastics are piled up in the environment all over the world, and about 2 million tons of waste plastics are thrown into the environment in China every year. Taking agricultural film for farmland as an example, the annual output of agricultural film in China reaches one million tons, increasing at the rate of 10% every year. No matter what crops are covered, the soil covered with plastic film has residual film. According to statistics, the annual residue of agricultural film in China is as high as 350,000 tons, and the residual film rate is 42%. A large number of residual films remain in the 0-30 cm plough layer of farmland. In other words, nearly half of the agricultural film remains in the soil, and food safety is a big hidden danger.
"Plastics are completely assimilated by microorganisms in the soil and decomposed into CO2 and water to realize inorganic mineralization, which may take 200-400 years, thus causing accumulation in the environment." Professor Yang Jun told the Yangcheng Evening News reporter. Since 2005, Yang Jun's team began to study the biodegradation of plastics. The main attack is the degradation of plastics such as polystyrene, which is the most difficult to degrade.
Scientists used several soil invertebrates, such as earthworms, millipedes, slugs and snails, to see if they could eat plastic. After feeding PVC, PE, PP and other 14C labeled plastics, the results showed that it could not be degraded.
Yang Jun believes that the idea of biodegradable plastics should not be limited to microorganisms. LEPIDOPTERA insects and termites can be considered. Marine moths and mussels can corrode polyethylene and submarine cables. It is also possible to isolate and clone key enzymes and their genes that can produce active groups from these organisms.
In the study of 20 14, Yang Jun's team found that wax insects (Indian corn borer larvae) can chew and eat polyethylene PE films, and two strains were isolated from the intestines of larvae, namely Enterobacter YT 1 and Bacillus YP 1, which can degrade PE films. Later, the research team found that Tenebrio molitor larva is a kind of animal that eats plastic badly. It is larger than wax worm (generally 35 mm long and 3 mm wide) and can use foam plastic as its only food. Tenebrio molitor has four life stages: egg, larva, pupa and adult.
Tenebrio molitor, also known as bread worm, belongs to COLEOPTERA, Tenebrionidae and Tenebrionidae (Tenebrionidae) in insect taxonomy. Tenebrio molitor originated in North America, and was introduced to China from the former Soviet Union in 1950s. It is known as "protein feed bank". Its dry products contain 30% fat, more than 50% protein and other elements such as phosphorus, potassium and iron. The dried Tenebrio molitor larvae contain about 40% protein, 57% pupae and 60% adults.
In China, Tenebrio molitor is actually similar to silkworm, which can be eaten directly, fried by human beings and used as feed. Scorpions, centipedes, geckos, snakes, tropical fish and goldfish fed with Tenebrio molitor not only grow fast and have high survival rate, but also have strong disease resistance and greatly improved fertility. It is very easy to raise Tenebrio molitor. Farmers can raise Tenebrio molitor with fresh oats, wheat bran and apples. Professor Yang Jun's team purchased polystyrene plastic raw materials from Sinopec Yanshan Branch, without additives and catalysts. However, polystyrene plastic samples labeled α- 13C and β- 13C were purchased from the United States. Tenebrio molitor was purchased from insect farms in Daxing, Beijing and Qinhuangdao, Hebei, and raised with grain. These insects are 3-4 years old (that is, molting 3-4 times).
These Tenebrio molitor are put in polypropylene plastic containers with foam blocks. The experimenter regularly measured the weight of foam blocks eaten by Tenebrio molitor, and the control group was Tenebrio molitor fed with conventional wheat bran. In the experiment, 500 Tenebrio molitor were fed with 5.8 grams of foam as the only food, and raised separately in the greenhouse under controlled conditions (25±65438±0°C, 80 2% humidity, 65438 06: 8 light/dark cycle). During the incubation period, the dead Tenebrio molitor was immediately removed.
In the experiment, Yang Jun and others fed Tenebrio molitor larvae with foamed plastic as a single food source. Compared with the larvae fed with wheat bran, it was found that the dry weight of larvae fed with wheat bran did not increase significantly (+33.6%) but only increased slightly by 0.2%, but did not decrease significantly (-24.
100 Tenebrio molitor can eat 34-39 mg of foam plastic every day. During the experimental period of 16 days, 47.7% of the foam plastics ingested by insects were converted into CO2. The residue (about 49.2%) is transformed into biodegradable particles similar to rabbit manure and excreted. Polystyrene plastics labeled with α- 13C or β- 13C were proved to be mineralized into carbon dioxide and lipids labeled with carbon 13. The retention time of polystyrene foam in the intestine of larvae is less than 24 hours.
1 month later, the larvae fed on polystyrene foam developed into crustaceans. Tenebrio molitor ate holes in the foam. After passing through the intestinal tract of worms, the chemical structure and composition of the ingested foam changed. By means of gel permeation chromatography (GPC), carbon 13 nuclear magnetic resonance spectrum and thermogravimetric Fourier transform infrared spectrum, it was confirmed that the long-chain polystyrene molecules in the intestine of larvae were broken and formed insect metabolites, which were excreted with feces.
In addition, a polystyrene-degrading bacterium YT2 (Microbacillus) was successfully isolated. YT2), which can be grown using polystyrene as the sole carbon source. The strain has been preserved in the General Microbiology Center and the National Gene Bank of China Microbial Species Preservation Management Committee, and it is the first reported polystyrene degrading strain in the world. The plastic we usually use is not a pure substance, it is made of many materials. Polymer (or synthetic resin) is the main component of plastic. In addition, in order to improve the properties of plastics, various auxiliary materials, such as fillers, plasticizers, lubricants, stabilizers, colorants, antistatic agents, etc. , must be added to polymer compounds to become a plastic with good performance.
Plastic additives, also known as plastic additives, are some compounds that must be added to improve the processability of polymers (synthetic resins) or the properties of resins themselves. For example, adding plasticizer to reduce the molding temperature of PVC resin and make the product soft; Another example is to add foaming agent to prepare lightweight, vibration-resistant, heat-insulating and sound-insulating foam plastics; The thermal decomposition temperature of some plastics is very close to the molding processing temperature, so it can't be molded without heating stabilizers. Therefore, plastic additives play a particularly important role in plastic molding. Prevent plastics from yellowing and cracking due to thermal oxidation during heating molding or high temperature use.
In addition to the above additives, there are flame retardants, foaming agents, antistatic agents, conductive agents, magnetic conductive agents, compatibilizers and so on. It can also be added to plastics. So as to meet different use requirements. According to the different use characteristics of various plastics, plastics are usually divided into three types: general plastics, engineering plastics and special plastics.
① Ordinary plastics
Generally, it refers to plastics with large output, wide use, good formability and low price. There are five kinds of general plastics, namely polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS) and acrylonitrile butadiene styrene (ABS). These five kinds of plastics account for the vast majority of plastic raw materials, and the rest can basically be classified as special plastic varieties, such as PPS, PPO, PA, PC, POM and so on. It is rarely used in daily life products, and is mainly used in high-end fields such as engineering industry and national defense technology, such as automobiles, aerospace, architecture, communications and other fields. Plastics can be divided into thermoplastics and thermosetting plastics according to their plasticity. Usually, thermoplastic products can be recycled, but thermosetting plastics can't. According to the optical properties of plastics, they can be divided into transparent, translucent and opaque raw materials, such as PS, PMMA, AS and PC, which belong to transparent plastics, while other plastics are mostly opaque plastics.
Properties and uses of common plastic varieties
1. Polyethylene: Commonly used polyethylene can be divided into low density polyethylene (LDPE), high density polyethylene (HDPE) and linear low density polyethylene (LLDPE). Among the three, HDPE has better thermal, electrical and mechanical properties, while LDPE and LLDPE have better flexibility, impact properties and film-forming properties. LDPE and LLDPE are mainly used in packaging film, agricultural film and plastic modification. And HDPE is widely used in films, pipes, daily necessities for injection and other fields.
2. Polypropylene: Relatively speaking, polypropylene has more varieties, more complex uses and more fields. The main varieties are homopolypropylene, block polypropylene (copp) and random polypropylene (rapp). According to different uses, homo is mainly used in drawing, fiber, injection molding, BOPP film and other fields.
3.PVC: Because of its low cost and self-flame retardant characteristics, it is widely used in the construction field, especially in sewer pipes, plastic steel doors and windows, plates, artificial leather and so on.
4. Polystyrene: As a transparent raw material, it is widely used, such as car lampshades, daily transparent parts, transparent cups and cans.
5.ABS is a widely used engineering plastic with excellent physical, mechanical and thermal properties. Widely used in household appliances, panels, masks, components, accessories and so on. Especially household appliances, such as washing machines, air conditioners, refrigerators, electric fans, etc. It is also widely used in plastic modification.
② Engineering plastics
Generally speaking, it refers to plastics that can withstand certain external forces, have good mechanical properties, high and low temperature resistance and good dimensional stability, and can be used as engineering structures, such as polyamide and polysulfone. Among engineering plastics, it can be divided into general engineering plastics and special engineering plastics. Engineering plastics can meet higher requirements in mechanical properties, durability, corrosion resistance and heat resistance, and can be processed more conveniently, and can replace metal materials. Engineering plastics are widely used in electronics, automobiles, construction, office equipment, machinery, aerospace and other industries. Replacing steel with plastic and wood with plastic has become an international fashion trend.
General engineering plastics include: polyamide, polyoxymethylene, polycarbonate, modified polyphenylene ether, thermoplastic polyester, ultra-high molecular weight polyethylene, methylpentene polymer, vinyl alcohol polymer and so on.
Special engineering plastics can be divided into crosslinking type and non-crosslinking type. Cross-linking types include: polyamino bismaleimide, polytriazine, cross-linked polyimide, heat-resistant epoxy resin and so on. Non-crosslinked ones include polysulfone, polyethersulfone, polyphenylene sulfide, polyimide, polyetheretherketone (PEEK) and so on.
③ Special plastics
Generally, it refers to plastics with special functions, which can be used in special application fields such as aviation and aerospace. For example, fluoroplastics and silicone resins have outstanding special functions such as high temperature resistance and self-lubrication, while reinforced plastics and foamed plastics have special properties such as high strength and high cushioning. These plastics belong to the category of special plastics.
A. reinforced plastics:
The raw materials of reinforced plastics can be divided into three types in appearance: granular (such as calcium-plastic reinforced plastics), fibrous (such as glass fiber or glass cloth reinforced plastics) and flaky (such as mica reinforced plastics). According to the materials, it can be divided into three types: cloth reinforced plastics (such as rag reinforced plastics or asbestos reinforced plastics), inorganic mineral filled plastics (such as timely or mica filled plastics) and fiber reinforced plastics (such as carbon fiber reinforced plastics).
B. Foam:
Foams can be divided into three types: hard, semi-hard and soft foams. Rigid foam has no flexibility and high compressive hardness. Only when the stress reaches a certain value can it be deformed, and it cannot be restored to its original state after the stress is released. Flexible foam has elasticity, low compressive hardness and easy deformation, and can be restored to its original state after stress relief with little residual deformation; The flexibility and other properties of semi-rigid foam are between rigid foam and soft foam. According to the different physical and chemical characteristics of various plastics, plastics can be divided into thermosetting plastics and thermoplastic plastics.
(1) Thermoplastics
Thermoplastic: refers to the plastic that will melt after heating, can flow into the mold for cooling, and will melt after heating; That is, it can be changed reversibly (liquid-solid) by heating and cooling, which is called physical change. The continuous service temperature of general-purpose thermoplastics is below 100℃, and it is also called the four general-purpose plastics together with polyethylene, polyvinyl chloride, polypropylene and polystyrene. Thermoplastics are divided into hydrocarbons, vinyl containing polar genes, engineering, cellulose and other types. It softens when heated, hardens when cooled, and can be softened and hardened repeatedly and maintain a certain shape. Soluble in a certain solvent, with melting and dissolving properties. Thermoplastics have excellent electrical insulation, especially polytetrafluoroethylene (PTFE), polystyrene (PS), polyethylene (PE) and polypropylene (PP). They have extremely low dielectric constant and dielectric loss, and are suitable for high-frequency and high-voltage insulation materials. Thermoplastics are easy to form and process, but have low heat resistance and are easy to creep. The degree of creep varies with load, ambient temperature, solvent and humidity. In order to overcome these weaknesses of thermoplastics and meet the application requirements in aerospace technology, new energy development and other fields, various countries are developing melt-moldable heat-resistant resins, such as polyetheretherketone (PEEK), polyethersulfone (PES), polyarylene sulfone (PASU) and polyphenylene sulfide (PPS). The composites based on them have high mechanical properties and chemical corrosion resistance, can be thermoformed and welded, and the interlaminar shear strength is better than that of epoxy resin. For example, the fatigue resistance of composites made of polyetheretherketone-based resin and carbon fiber is higher than that of epoxy/carbon fiber. It has good impact resistance, creep resistance at room temperature and good processability, and can be used continuously at 240 ~ 270℃. It is an ideal insulating material with high temperature resistance. The composites made of polyethersulfone resin and carbon fiber have high strength and hardness at 200℃, and can still maintain good impact resistance at-100℃. It is non-toxic, non-flammable, with the least smoke and good radiation resistance. It is expected to be used as a key component of spacecraft and can also be shaped into a radome.
Formaldehyde crosslinked plastics include phenolic plastics and amino plastics (such as urea formaldehyde, melamine formaldehyde, etc.). ). Other crosslinked plastics include unsaturated polyester, epoxy resin, allyl phthalate resin, etc.
(2) Thermosetting plastics
Thermosetting plastics refer to plastics that can be cured or have insoluble (melting) characteristics under heating or other conditions, such as phenolic plastics and epoxy plastics. Thermosetting plastics are divided into formaldehyde crosslinking type and other crosslinking types. After thermal processing, an insoluble cured product is formed, and its resin molecules are crosslinked into a network structure from a linear structure. If the heat intensifies, it will decompose and destroy. Typical thermosetting plastics are phenolic, epoxy, amino, unsaturated polyester, furan, polysiloxane and other materials, as well as relatively new polypropylene terephthalate plastics. They have the advantages of high heat resistance and difficult deformation when heated. The disadvantage is that the mechanical strength is generally not high, but it can be improved by adding fillers to make laminated materials or molded materials.
Thermosetting plastics made of phenolic resin as the main raw material, such as phenolic molding compound (commonly known as bakelite), are durable, dimensionally stable and resistant to other chemicals except strong alkali. Various fillers and additives can be added according to different purposes and requirements. For varieties with high insulation performance, mica or glass fiber can be used as filler; For heat-resistant varieties, asbestos or other heat-resistant fillers can be used; If earthquake-resistant varieties are needed, various suitable fibers or rubbers can be used as fillers and some toughening agents to make high-toughness materials. In addition, phenolic resin modified by aniline, epoxy, polyvinyl chloride, polyamide and polyvinyl acetal can be used to meet the requirements of different uses. Phenolic resin can also be used to make phenolic laminates, which are characterized by high mechanical strength, good electrical properties, corrosion resistance and easy processing, and are widely used in low-voltage electrical equipment.
Amino plastics include urea-formaldehyde, melamine formaldehyde, urea melamine formaldehyde and so on. They have the advantages of hard texture, scratch resistance, colorless and translucent, and can be made into colorful products by adding pigments, commonly known as electric jade. Because it is oil-resistant and unaffected by weak alkali and organic solvents (but not acid-resistant), it can be used at 70℃ for a long time and at 1 10 ~ 120℃ for a short time, and can be used in electrical products. Compared with urea-formaldehyde plastics, melamine-formaldehyde plastics have higher hardness, better water resistance, heat resistance and arc resistance, and can be used as arc-resistant insulating materials.
There are many kinds of thermosetting plastics with epoxy resin as the main raw material, of which about 90% is bisphenol A epoxy resin as the main raw material. It has excellent adhesion, electrical insulation, heat resistance and chemical stability, small shrinkage and water absorption, good mechanical strength and so on.
Unsaturated polyester and epoxy resin can be made into FRP with excellent mechanical strength. For example, the glass fiber reinforced plastic with unsaturated polyester has good mechanical properties and low density (only 1/5 for steel and 1/4 for aluminum), and is easy to be processed into various electrical parts. The electrical and mechanical properties of plastics made of diallyl terephthalate resin are better than phenolic and amino thermosetting plastics. Small hygroscopicity, stable product size, good formability, acid and alkali resistance, boiling water resistance and some organic solvents. Molding compound is suitable for manufacturing parts with complex structure, high temperature resistance and high insulation. Generally, it can be used for a long time in the temperature range of -60 ~ 180℃, and its heat resistance can reach F to H, which is higher than that of phenolic resin and aminoplast.
Silicone plastics with polysiloxane structure are widely used in electronic and electrical technology. Silicone laminated plastics mostly uses glass cloth as reinforcing material; Molded plastics in silicone resin are mostly filled with glass fiber and asbestos, which are used to make parts of high temperature, high frequency or submersible motors, electrical appliances and electronic equipment. This kind of plastic is characterized by low dielectric constant and tgδ value, and is less affected by frequency. It is used in electrical and electronic industries to resist corona and arc. Even if the discharge leads to decomposition, the product is silicon dioxide instead of conductive carbon black. This material has outstanding heat resistance and can be used continuously at 250℃. The main disadvantages of polysiloxane are low mechanical strength, low adhesion and poor oil resistance. Many modified silicone polymers, such as polyester modified silicone plastics, have been developed and applied to electrical technology. Some plastics are both thermoplastic and thermosetting. For example, polyvinyl chloride (PVC) is usually a thermoplastic. Japan has developed a new type of liquid PVC, which is thermosetting and its molding temperature is 60 ~ 140℃. A plastic called Lundex in the United States has both the characteristics of thermoplastic processing and the physical properties of thermosetting plastics.
① Hydrocarbon plastics. Belonging to nonpolar plastics, it can be divided into crystalline and amorphous. Crystalline hydrocarbon plastics include polyethylene and polypropylene, and amorphous hydrocarbon plastics include polystyrene.
② Vinyl plastics containing polar genes. Except fluoroplastics, most of them are amorphous transparent body, including polyvinyl chloride, polytetrafluoroethylene and polyvinyl acetate. Most vinyl monomers can be polymerized by free radical catalysts.
③ Thermoplastic engineering plastics. It mainly includes polyformaldehyde, polyamide, polycarbonate, ABS, polyphenylene ether, polyethylene terephthalate, polysulfone, polyethersulfone, polyimide, polyphenylene sulfide and so on. Polytetrafluoroethylene. Modified polypropylene and the like are also included in this range.
④ Thermoplastic cellulose plastics. Mainly includes cellulose acetate, cellulose acetate butyrate, cellophane, cellophane, etc. According to different plastic molding methods, it can be divided into film pressing, lamination, injection, extrusion, blow molding, plastic casting and reaction injection.
Film-pressed plastics are mostly plastics with physical properties and processability similar to those of general solid plastics; Laminated plastics refers to the fiber fabric impregnated with resin, which is combined into a whole through superposition and hot pressing; Injection molding, extrusion and blow molding are mostly plastics with similar physical properties and processability to general thermoplastics; Casting refers to the liquid resin mixture that can be hardened into a certain shape product by pouring it into the mold without pressure or with a little pressure, such as MC nylon; Reaction injection plastic is a kind of plastic, such as polyurethane, which injects liquid raw materials into the film cavity under pressure to make it react and cure into a certain shape of products.