Plastic is a high molecular weight mixture polymerized by polyaddition or polycondensation reaction with a monomer as raw material. Its resistance to deformation is average, between fiber and rubber. Agents, lubricants, dyes and other additives. The main component of plastic is resin. Resin refers to a polymer compound not mixed with various additives. The basic properties of plastics are mainly determined by nature of resin, but additives also play an important role.
Plastic is an important organic synthetic polymer material with a wide range of applications. However, "white pollution" caused by plastic waste is becoming more and more serious. If we can learn more about plastics, it will not only help us use plastic products scientifically, but also make it easier to sort and recycle plastics, and also effective control. and reduce white pollution.
1. Main characteristics of plastic
1. Most plastics are chemically stable, have high corrosion resistance and do not react with acids and alkalis.
2. Wide range of applications, cheap price and low production costs. Some plastics dissolve easily in solvents.
3. Durable, waterproof, lightweight and durable, with good wear and impact resistance.
4. It is easy to process and can be mass-produced, has low cost, and is easily molded into various shapes.
5. Most of them are good insulators with low thermal conductivity, and some are resistant to high temperatures.
6. Glossy, partially transparent or translucent, easy to color.
⒎ Can be used to prepare fuel oil and fuel gas to reduce crude oil consumption.
1. When recycling plastic waste, it is difficult to sort and is not economically viable.
2. Most plastics have low heat resistance, high thermal expansion rates, burn easily, and emit toxic gases when burned. For example, burning polystyrene produces toluene. Small amounts of this substance can cause blindness and symptoms such as vomiting if inhaled; burning PVC also releases toxic hydrogen chloride gas. In addition to combustion, it is a high-temperature environment. , which will lead to decomposition of plastic and formation of toxic components such as benzene, etc.
3. Plastics are made from petroleum products, and petroleum resources are limited.
4. Plastic has been rotting underground for hundreds of years.
5. Plastics have poor heat resistance, poor dimensional stability, and are easily deformed. Most plastics have poor low temperature resistance, become brittle at low temperatures, and age easily.
6. Due to unnatural degradability of plastic, it has become enemy No. 1 of man, which caused tragedy of deathmany animals. For example, monkeys, pelicans, dolphins and other animals in zoo will accidentally swallow plastic bottles thrown away by tourists and die painfully due to indigestion, various indigestible substances from plastic were found in intestines of many dead seabirds.
Secondly, structure and types of plastic
Plastic has two main structures:
The first type is a linear structure, and a polymer compound with such a structure is called a linear polymer compound;
The second type is structure of body, and combination of macromolecules with such a structure is called association of macromolecules of body.
Two different structures showing two opposite properties. Linear structure, heating can melt, less hardness and brittleness. The body structure is relatively hard and brittle.
Products made from linear polymers are classified as thermoplastics, while products made from bulk polymers are thermosets.
Usually, thermoplastic products can be recycled, but thermosetting plastics cannot. According to optical properties, plastics can be divided into transparent, translucent and opaque raw materials.
Thermoplastics: refers to plastics that melt after heating, can flow into a mold to form after cooling, and then melt after reheating; heating and cooling can be used to produce reversible changes (liquid ←→ hardness). so-called physical changes. It is soluble in some solvents and has ability to melt and dissolve.
Polyethylene, polyvinyl chloride, polypropylene and polystyrene are collectively known as four general purpose plastics. The continuous use temperature of general purpose thermoplastics is below 100°C. Thermoplastics have excellent electrical insulation, especially polytetrafluoroethylene (PTFE), polystyrene (PS), polyethylene (PE), polypropylene (PP) all have extremely low dielectric constant and dielectric loss. and are suitable for high frequency and high voltage insulation materials.
Thermoplastics are easy to mold and process, but they have low heat resistance and are prone to creep. The degree of creep depends on load, ambient temperature, solvent and humidity.
2. Thermoset plastic
Thermosetting plastics refer to plastics that can harden or become insoluble (melting) properties when heated or otherwise, such as phenolic plastics, epoxy plastics, etc.
Thermosetting plastics are divided into two types: formaldehyde crosslinking and other type crosslinking.
Common thermoset plastics include phenolic, epoxy, amino, unsaturated polyester, furan, polysiloxane and other materials, as well as newer poly(dipropylene phthalate) plastics. They have advantageHigh temperature resistance properties and not easily deformed when heated. The disadvantage is that mechanical strength is generally low, but its mechanical strength can be improved by adding fillers to make laminated or molded materials.
Formaldehyde-crosslinked plastics include phenolic plastics and amino plastics (such as urea-formaldehyde-melamine-formaldehyde, etc.). Other crosslinked plastics include unsaturated polyesters, epoxy resins, diallyl phthalate resins, and like.
They are durable, dimensionally stable, moldable and resistant to chemicals other than strong alkalis. Scratch resistance, colorless, translucent, oil resistance, water resistance, arc resistance, excellent adhesion, electrical insulation, heat resistance and chemical resistance, small shrinkage and water absorption, good mechanical strength, etc.
According to different usage characteristics of different plastics, plastics are generally divided into three types: general purpose plastics, engineering plastics, and specialty plastics.
Generally refers to plastics with high yield, wide application, good moldability and low price. There are five general purpose plastics, namely polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), and acrylonitrile butadiene styrene copolymer (ABS).
Common engineering plastics include: polyamide, polyoxymethylene, polycarbonate, modified polyphenylene ether, thermoplastic polyester, ultra high molecular weight polyethylene, methyl pentene polymer, vinyl alcohol copolymer, etc.
Generally refers to plastics that can withstand certain external forces, have good mechanical properties, high and low temperature resistance, good dimensional stability, and can be used as engineering structures, such as polyamide and polysulfone.
Engineering plastics fall into two categories: general engineering plastics and specialty engineering plastics. Engineering plastics have higher requirements for mechanical properties, durability, corrosion resistance, heat resistance, etc., they are more convenient to process and can replace metal materials. Engineering plastics are widely used in electronics, automobiles, construction, office equipment, mechanical engineering, aerospace, etc. The replacement of steel and wood with plastics has become an international trend.
⑶ Special Heads
Usually refers to plastics that perform specific functions and can be used in specific applications such as aviation and space. For example, fluoroplastics and silicones have special features such as outstanding high temperature resistance and self-lubrication, andreinforced plastics and foams have special properties such as high strength and high shock absorption. These plastics are categorized as specialty plastics.
Special engineering plastics are divided into crosslinked and non-crosslinked.
Crosslinked types include: polyaminobismaleimide, polytriazine, crosslinked polyimide, heat resistant epoxy, etc.
Non-crosslinked types include: polysulfone, polyethersulfone, polyphenylene sulfide, polyimide, polyetheretherketone (PEEK), etc.
① Reinforced plastic:
Reinforced plastic raw materials can be divided into three types according to shape: granular (such as calcium-reinforced plastics), fibrous (such as fiberglass or fiberglass reinforced plastics), and sheet (such as mica-reinforced plastics).
Foam foams can be divided into three types: rigid, semi-rigid and flexible foams.
Rigid foam has no flexibility and compressive rigidity is very high. It will only deform when a certain stress is reached and will not return to its original shape when stress is removed;
Flexible foams have high flexibility, low compressive hardness and easy deformation. After stress is removed, they can return to their original shape, and permanent deformation is small;
Semi-rigid foams are intermediate in flexibility and other properties between rigid and flexible foams.
An appropriate foaming agent is added to foaming material so that plastic has a microporous structure. Almost all thermoplastics and thermoplastics can be turned into foam. According to cellular structure, foam is divided into open-cell (most of pores are interconnected) and closed-cell (most of pores are separated from each other), which is mainly determined by manufacturing method (divided into chemical foaming, physical foaming and mechanical foaming) Foaming) is determined.
4. Other new plastics
⑴High thermal conductivity bioplastics
NEC has recently developed a plant-based bioplastic with thermal conductivity comparable to that of stainless steel. The company mixed carbon fibers a few millimeters long and 0.01 mm in diameter and a special adhesive with polylactic acid resin from corn to create a new type of bioplastic with high thermal conductivity. With addition of 10% carbon fiber, thermal conductivity of bioplastic is comparable to that of stainless steel, with addition of 30% carbon fiber, thermal conductivity of bioplastic is twice that of stainless steel, and density is only 1/5. from stainless steel.
In addition to good thermal conductivity, this bioplastic also has advantages of light weight, easy molding and low environmental pollution. It can be used to producethe thin and light outer frames of electronic products such as computers and mobile phones.
⑵ Color-changing plastic film
The University of Southampton in UK and Institute for Plastics Research in Darmstadt, Germany, have jointly developed a color-changing plastic film. Combining natural and artificial optical effects, film is actually a new way to accurately change color of objects. This color-changing plastic film is a plastic opal film composed of plastic spheres stacked on top of each other in three-dimensional space, and contains tiny carbon nanoparticles in middle of plastic spheres, so that light is not only reflected between plastic spheres and surrounding materials reflected from edge areas between plastic spheres, as well as from surface of carbon nanoparticles filled between these plastic spheres. This greatly deepens color of film. As long as volume of plastic granules is controlled, it is possible to produce light substances that scatter only certain spectral frequencies.
⑶ Plastic Blood
Researchers at University of Sheffield in United Kingdom have developed artificial "plastic blood" that looks like a thick paste. If it is dissolved in water, it can be used to transfuse blood to patients. It can be used as a blood substitute in emergency procedures. This new type of artificial blood is made up of plastic molecules. There are millions of plastic molecules in a piece of artificial blood. These molecules are similar in size and shape to hemoglobin molecules. They can also carry iron atoms and transport oxygen throughout body. like hemoglobin. Since raw material is plastic, this kind of artificial blood is portable and easy to carry, does not require refrigeration, has a long shelf life, higher work efficiency than real artificial blood, and is cheaper.
⑷New Bulletproof Plastic
In 2013, a Mexican research team developed a new type of bulletproof plastic that can be used to make bulletproof glass and bulletproof clothing, and is only 1/5 to 1/7 weight of traditional materials. It is a specially treated plastic substance that is more bulletproof than conventional engineering plastics. After being hit by a bullet, this new material deforms temporarily, but soon returns to its original shape and can continue to be used. In addition, this new material makes it possible to evenly distribute impact force of a bullet, thereby reducing damage to human body.
⑸ Vehicle Noise Reducing Plastics
Polymers Group Inc. (PGI) used renewable polypropylene and polyethylene terephthalate to create a new base material for car injection molding.impact parts to reduce noise. This material is mainly used in body upholstery and wheel arches to create a barrier layer that can absorb sound in car interior and reduce noise by 25% to 30%. PGI has developed a special one-step manufacturing process that seamlessly combines recycled materials. and raw materials, and combine two materials into one through lamination and needle punching.
3. Processing and application of plastic
The plastic molding process is process of making final plastic products from polymers produced by synthetic resin manufacturers.
Processing methods (commonly referred to as primary plastics recycling) include compression molding (compression molding), extrusion molding (extrusion molding), injection molding (injection molding), blow molding (hollow molding), calendering, etc. < /p>
According to different molding methods of various plastics, they can be divided into various types, such as film pressing, lamination, injection molding, extrusion, blow molding, plastic injection and injection molding plastics.
In order to improve performance of plastics, it is necessary to add various auxiliary materials such as fillers, plasticizers, lubricants, stabilizers, dyes, antistatic agents, etc. to polymer compounds to become plastics with good performance. .
At same time, fire retardant, foaming agent, conductive agent, magnetic permeability agent, compatibilizer, etc. are added to meet different usage requirements. For example, plasticizers are added to lower molding temperature of PVC resin and make product soft; foaming agents are added to obtain light, vibration-resistant, heat-insulating and sound-proof foams; thermal decomposition temperature of some plastics. It is very close to molding processing temperature and cannot be molded without addition of a heat stabilizer. Thus, plastic additives play a particularly important role in plastic injection process.
Packaging is first use of plastic
Plastic packaging materials have advantages of light weight, high strength, good impact resistance, transparency, moisture resistance, beautiful appearance, stable chemical properties, good strength and corrosion resistance, and they have widely replaced metal, wood, paper, glass. , leather, etc. Thus, plastic packaging has played an indispensable role in reducing pressure on my country's resources and energy.
Plastic flexible packaging, woven bags, hollow containers, return boxes, etc., various mineral products, chemical products, raw salt, grains, sugar, food and medicine, cotton and wool, etc., as well as drinks, detergentsproducts, cosmetics, composite films, packaging film, etc. have a strong demand for plastic packaging materials.
Agricultural applications: plastic consumption has skyrocketed
Agro-plastic products are indispensable materials for development of modern agriculture. They are an indispensable technical measure to withstand natural disasters and obtain stable, high-yielding, high-quality and high-performance crops. They are widely used in China's agriculture, forestry, livestock, fisheries and agriculture have become second largest consumption area of plastic products after packaging industry.
Fourth, white pollution and countermeasures
The rapid development of plastics industry and widespread use of plastic products have penetrated every corner of society. From industrial production to daily necessities, plastic products are everywhere. But it is often thrown away after being consumed once, and plastic waste is slowly pouring into us. Every year, 40 million tons of plastic waste accumulate in environment around world, and about 2 million tons of plastic waste are released into environment in China.
"White pollution" caused by plastic waste causes headaches. Plastics do not easily decompose, do not rot, and cannot be effectively recycled. It can take 200 to 400 years for plastic to be fully assimilated by microorganisms in soil and decompose into CO2 and water to achieve inorganic mineralization, resulting in accumulation in environment. Taking agricultural film for farmland as an example, annual production of agricultural film in my country has reached one million tons, and it is increasing at a rate of 10% per year. soils have a residual film. The level of residual film reached 42%, and a large amount of residual film remained in arable layer of 0-30 cm of farmland. That is, almost half of agricultural film remains in soil.
Plastic waste is becoming an increasingly prominent global environmental problem, putting great pressure on human environment and seriously affecting our health and environment. Therefore, recycling of plastic waste is inevitable.
The study showed that waxworms (Indian flour moth larvae) can chew and eat polyethylene polyethylene films, and two strains of enterobacteria YT1 and Bacillus YP1 were isolated from intestines of larvae, which can degrade polyethylene films. Tenebrio molitor larvae, even more aggressive plastic eaters, can consume Styrofoam as their only food. This discovery provides ideas for solving global problem of plastic pollution.
In short, development of plastic technology is changing rapidly. With development of technology, developmentnew materials are being developed for completely new applications. hair. It is hoped that in near future plastics, once modified, can be used more widely and can even replace steel and other materials without polluting environment.