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With China Plastic Machinery
Plastics are materials with polymers as the main components, which can be molded into a certain shape under certain conditions and keep their shape unchanged at room temperature. It is customary to include semi-finished plastics, such as compression molding powder. As the basic component of plastics, polymers not only determine the type of plastics but also determine the main properties of plastics. Generally speaking, the cohesive energy of plastic polymers is between fiber and rubber, and the use temperature range is between its brittle temperature and glass transition temperature.
Plastics are made of the main raw materials of petroleum or natural gas. They are a large class of high molecular polymers, which are the general term for polyvinyl chloride, polystyrene, polyethylene, polypropylene, etc., which often contain additives such as plasticizers and colorants. It has the properties of light weight, corrosion resistance, heat resistance, impact resistance, tensile resistance, bending resistance, heat preservation and energy saving. It is widely used in all walks of life, such as daily necessities (beverage bottles, food packaging bags, etc.) in life, can be used to manufacture various parts, equipment, materials, etc. in industry, and can be used to manufacture medical devices, etc. in medicine.
Plastic pollution, also known as white pollution, is caused by a large amount of waste agricultural films, packaging plastic films, plastic bags and disposable plastic tableware (hereinafter collectively referred to as plastic packaging) being discarded as garbage after use, causing great damage to the landscape and ecological environment. Since most of the waste plastic packaging is white, the environmental pollution caused by it is called "white pollution". White pollution has two negative impacts on the environment: "visual pollution" and "potential harm".
As early as the 19th century, people had used natural resins such as pine, casein and shellac to make goods. In the 1840s, British chemist and inventor Alexander Parkes developed the earliest synthetic material cellulose nitrate and named it "Parkesine". He demonstrated this material in 1862. In 1869, American chemist John Wesley Art applied for a patent for a cellulose nitrate synthesis process. He nitrated natural cellulose and used camphor as a plasticizer to make the world's first plastic variety, called Celluloid, which began the history of human use of plastics.
In 1907, Belgian-born chemist Leo Baekeland discovered Bakelite, which found use in the then-nascent telecommunications industry. Its use peaked during the Great Depression, thanks to cheap, brightly colored plastics available when money was scarce.
In the early 1920s, German chemist Hermann Staudinger made a startling discovery that would change the nature of the plastics industry forever. He discovered that plastics were made up of thousands of molecular chains. The development of these "superpolymers" also underpinned the invention of many new products over the next few decades. By the late 1920s, polyvinyl chloride (PVC) was first used as an insulator for cables, and its popularity quickly grew. Today, PVC comes in two forms - rigid and flexible - the latter of which is widely used in packaging.
By the end of World War II, the family of plastics included polyethylene (PE), polystyrene (PS), plexiglass (Perspex), polyurethane (PU), polypropylene (PP), acrylonitrile-butadiene-styrene copolymer (ABS) and polyethylene terephthalate (PET), many of which are still widely used today. Around 1952, Italians Giulio Natta and Karl Ziegler invented polypropylene - an achievement that won them the Nobel Prize in Chemistry. Joe Colombo was one of the representatives who tried the latest production processes and newly developed plastics, including fiberglass, ABS, polyvinyl chloride and polyethylene.
Plastics are high-molecular organic materials with resin as the main component, adding appropriate additives (such as fillers, plasticizers, curing agents, stabilizers, lubricants, colorants, etc.), molded into a certain shape under a certain temperature and pressure, and can maintain the given shape at room temperature. Resin refers to an organic polymer that usually has a transformation or melting range when heated, has fluidity under the action of external forces when transformed, and is solid, semi-solid or liquid at room temperature. It is the most basic and most important component of plastics. Broadly speaking, any polymer used as the basic material of plastics in the plastics industry can be called a resin. Resins include natural resins and synthetic resins. Synthetic resins are mainly used in the modern plastics industry. Commonly used synthetic resins include polyethylene, polypropylene, polyvinyl chloride, polystyrene, phenolic resin, unsaturated polyester resin and epoxy resin. At room temperature, they can be solid, semi-solid or liquid. Solid or semi-solid resins become flowable melts or liquids when heated. Most plastics are composed of carbon compounds associated with large molecules (macromolecules). In addition to carbon, plastics also contain hydrogen, and some plastics also contain oxygen, nitrogen, chlorine and fluorine. Plastics are made from the main raw materials of petroleum or natural gas, and their manufacturing is divided into two steps: the synthesis of active semi-finished products. This type of semi-finished product is mostly composed of single molecules, so it is called a monomer. Thousands of single molecules are connected to form macromolecules (macromolecules). The substance produced here is called a polymer. A single molecule can form macromolecular polymerization, polycondensation and polyaddition reactions according to different reaction types.
Plastics are mainly composed of polymers and belong to high molecular compounds, referred to as polymers. Each polymer contains one or several monomers, which are arranged in a certain way. First, they are arranged into many links of the same structure, and then connected into a molecular chain through chemical bonds. If a polymer is composed of molecular chains, it is called a linear polymer; if there are some short chains between the chains of macromolecules to connect them, it is called a bulk polymer. In addition, there is a network polymer, which is between the linear and bulk structures. There is actually no strict difference from the bulk structure, but the short chains cross-linked between the molecular chains are relatively loose.
General-purpose plastics refer to a type of plastic that can generally only be used as non-structural materials, with large output, wide applications, low prices and ordinary performance. The main varieties are polyethylene, polypropylene, polyvinyl chloride, phenolic plastics and amino plastics, accounting for more than 75% of the total plastic production.
Engineering plastics refer to a type of plastic that can be used as engineering structural materials, has excellent mechanical properties, and can withstand mechanical stress and harsh chemical and physical environments in a wide temperature range. There are mainly various reinforced plastics such as polyamine (nylon), polycarbonate acetal, ABS, polyphenylene ether, and polyurethane.
Compared with general plastics, engineering plastics have a small output and a higher price, but have excellent mechanical properties, electrical properties, chemical properties, wear resistance, heat resistance, corrosion resistance, self-lubrication and dimensional stability, and have certain metal properties, so they can replace some metal materials to manufacture structural parts and transmission structural parts.
Functional plastics refer to plastics that are used in special environments and have special properties in a certain aspect. There are mainly medical plastics, photosensitive plastics, magnetic plastics, high heat-resistant plastics and high-frequency insulating plastics. This type of plastic has a small output, is relatively expensive, and has excellent performance.
According to the chemical composition of plastics, commonly used ones include polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), polycarbonate (PC), polyamide (PA), etc.
The properties of polyethylene (PE) vary from variety to variety, mainly depending on the molecular structure and density. Different production methods can produce products of different densities. Polyethylene can be processed by the molding method of general thermoplastics, mainly used to make films, containers, pipes, monofilaments, wires and cables, daily necessities, etc., and can be used as high-frequency insulation materials for televisions, radars, etc.
Polypropylene (PP) is a semi-crystalline material. It is harder and has a higher melting point than PE. Polymer-type PP materials have a lower heat distortion temperature (100¡ãC), low transparency, low gloss, and low rigidity. Generally, PP is modified by adding glass fiber, metal additives or thermoplastic rubber. Polypropylene products can be steam sterilized and are suitable for medical devices and mechanical parts. Polypropylene can also be used as anti-corrosion materials such as chemical containers and surface coatings, and is widely used in the electrical industry. The non-toxicity of polypropylene also makes it possible to use it in the packaging of food, medicines and daily groceries. For example, it can be used to make clothing linings, blankets, cotton products, work clothes, carpets, filter cloths, waterproof cloths, etc. It is also used in the shoemaking industry.
Polyvinyl chloride (PVC) is actually a vinyl polymer, and its material is a non-crystalline material. In actual use, stabilizers, lubricants, auxiliary processing agents, colorants, impact agents and other additives are often added to PVC materials. It is non-flammable, high-strength, resistant to climate change and has excellent geometric stability. PVC has strong resistance to oxidants, reducing agents and strong acids. PVC materials are extremely widely used, and have good processing performance, low manufacturing cost, corrosion resistance, insulation and other good characteristics.
PVC materials are mainly used to make: Prewin PVC cards, PVC labels, PVC iron wire, PVC curtains, PVC plastic-coated welded mesh, PVC foam boards, PVC ceilings, PVC water pipes, PVC skirtings, etc., as well as threading tubes, cable insulation, plastic doors and windows, plastic bags, etc. PVC products can be seen everywhere in our daily lives. PVC is used to make various imitation leathers, used in luggage bags, sports products such as basketballs, footballs and rugby. It can also be used to make belts for uniforms and special protective equipment.
Polystyrene (PS) is colorless and transparent, can be freely colored, and its relative density is second only to PP and PE. It has excellent electrical properties, especially good high-frequency characteristics. In addition, it is the strongest among all plastics in terms of light stability. The most important feature of polystyrene is that its thermal stability and mobility when molten are very good, so it is easy to mold and process, especially injection molding, and is suitable for mass production. It is often used to make extruded boards. Polystyrene has a universal use value in the light industry market and general industrial decoration, lighting indication and other places. In the electrical field, it is a good insulating material and heat insulation material. For example, it can be used to make various instrument housings, lampshades, chemical instrument parts, optical instrument parts, transparent models, and frozen insulation layers. Polystyrene foam can be used as insulation materials for refrigerators, trains, ships, buildings, etc.
Polycarbonate (PC) is colorless and transparent, heat-resistant, impact-resistant, flame-retardant BI grade, and has good mechanical properties within the normal use temperature. Compared with polymethyl methacrylate with similar performance, polycarbonate has good impact resistance, high refractive index and good processing performance. Polycarbonate is widely used in the fields of machinery industry, electronic and electrical industry, aviation industry, optics and lighting. It can be used to make various gears, motor parts, aviation materials, transparent organic glass, etc.
Polyamide (PA, commonly known as nylon) is the first resin developed by DuPont in the United States for fiber, and it was industrialized in 1939. PA has good comprehensive properties, including mechanical properties, heat resistance, wear resistance, chemical resistance and self-lubrication, and has a low friction coefficient, certain flame retardancy, and is easy to process. It is suitable for filling and reinforcing modification with glass fiber and other fillers to improve performance and expand the scope of application. Polyamide is widely used in various machinery, transportation, electronics and electrical and daily industrial parts and resin coatings, films, etc. due to its good mechanical properties, electrical properties, good wear resistance, oil resistance, self-lubrication and processing properties.
Plastics have properties such as light weight, corrosion resistance, heat resistance, impact resistance, tensile resistance, bending resistance, thermal insulation and energy saving, but plastics are organic materials and have various shortcomings, such as low mechanical strength and hardness, poor antistatic properties, etc. Therefore, in the production process of plastics, these performance shortcomings can be improved by adding various additives.
Fillers, also known as fillers, can improve the strength and heat resistance of plastics and reduce costs. Fillers can be divided into two categories: organic fillers and inorganic fillers. The former include wood flour, rags, paper and various fabric fibers, while the latter include glass fiber, diatomaceous earth, asbestos, carbon black, etc.
Plasticizers can increase the plasticity and softness of plastics, reduce brittleness, and make plastics easy to process and shape. Plasticizers are generally high-boiling organic compounds that are miscible with resins, non-toxic, odorless, and stable to light and heat. Commonly used plasticizers include camphor, phosphate esters, benzophenone, etc.
In order to prevent synthetic resins from being decomposed and damaged by light and heat during processing and use, and to extend their service life, stabilizers should be added to plastics. Commonly used stabilizers include stearates, epoxy resins, etc.
Colorants can give plastics a variety of bright colors, and organic dyes and inorganic pigments are commonly used as colorants.
The role of lubricants is to prevent plastics from sticking to metal molds during molding, and at the same time make the surface of plastics smooth and beautiful. Commonly used lubricants include stearic acid and calcium magnesium salts.
The role of antioxidants is to prevent plastics from being heated and oxidized during molding or high-temperature use, which causes plastics to turn yellow and crack.
A large amount of waste agricultural films, packaging plastic films, plastic bags and disposable plastic tableware (hereinafter collectively referred to as plastic packaging) are discarded as garbage after use, causing great damage to the landscape and ecological environment. Since most of the waste plastic packaging is white, the environmental pollution caused by it is called "white pollution". White pollution has two negative effects on the environment: "visual pollution" and "potential harm".
Visual pollution refers to the damage to the city's appearance caused by waste plastics scattered in the environment. Waste plastics scattered in large cities, tourist areas, water bodies, and railways bring adverse stimulation to people's vision and affect the overall beauty of cities and scenic spots. For example, disposable foamed plastic tableware scattered in the natural environment, on both sides of railways, rivers and lakes, and ultra-thin plastic bags flying in the sky or hanging on branches, bring adverse stimulation to people's vision.
Potential hazards refer to the long-term and deep-seated environmental problems caused by the difficulty of degradation of plastic waste after entering the natural environment. Plastic mulch waste remains in the soil over a large area and accumulates for a long time, causing soil compaction, affecting the absorption of nutrients and water by crops, and leading to reduced crop yields; plastic waste abandoned on land or in water bodies is eaten by animals as food, causing animal death; plastic waste entering domestic garbage is light in weight and large in volume, making it difficult to handle. If it is landfilled, it will occupy a large amount of land and will be difficult to degrade for a long time. The plastic products widely used in the market are non-degradable plastics, made of expanded polystyrene, polyethylene or polypropylene, with a molecular weight of more than 20,000. Only when the molecular weight is reduced to below 2,000 can it be used by microorganisms in the natural environment and turned into water and other organic matter, and this process takes 200 years; if it is landfilled, it will affect the absorption of nutrients and water by crops, resulting in reduced production; burning it will release a variety of chemical toxic gases. Among them, there is a compound called dioxin, which is extremely toxic. Even if it is ingested in a small amount, it can cause deformities or death in birds and fish, causing damage to the ecological environment and also causing great harm to humans.
The recycling of plastic waste can partially solve the problem of environmental pollution, save energy and raw materials, etc., and is the preferred solution for treating plastic waste. Recycling and reuse generally includes three steps: collection, separation, and reuse.
Collection work is a social and technical task. The first is legislation, and the second is to establish a recycling center and organize a recycling team. Many countries have formulated corresponding plastic recycling regulations and policies. The new U.S. Environmental Protection Agency's urban waste countermeasures set a national goal of recycling at a rate of 1.5% per year. A supplementary recommendation to the current "Resource Conservation and Recovery Program" stipulates that the recycling rate must reach 1.5% per year. Japanese law clearly stipulates that residual mulch is not allowed in the soil after harvest. The French government proposed a deposit system for drinking plastic bottles. The Italian government has legislated that beverage bottle recycling targets must be met, and requires manufacturers and packagers to invest in recycling companies. Those who fail to meet the above targets will be subject to heavy taxes.
Separation is an important process for the recycling of waste plastics. The main separation technologies are divided into manual separation and equipment separation. Manual separation is to use manual methods to carry out sorting. The general steps are: ¢Ù Remove metal and non-metal impurities first; ¢Ú Classify waste plastics of different qualities; ¢Û Sorting according to the color depth and aging degree of the products. Equipment separation methods include wind screening method, specific gravity separation method, etc. The above two separation methods are used in combination.
There are three types of waste plastic recycling technologies. ¢Ù Simple recycling mainly uses the scraps generated in the production process of resin production plants and plastic product factories, and can also include those disposable wastes that are easy to clean and select, such as polyphenol beverage bottles, fast food boxes, milk packaging bags, etc. This part of the waste material is characterized by being relatively clean and simple in composition, and can be reused in a relatively simple process route and equipment. ¢Ú The waste used for composite recycling mainly comes from waste plastics collected from different channels after being used in commodity circulation. After these plastics are reduced in volume, shredded, cleaned and separated or sorted, they must be crushed again to enter plasticizing machinery or granulating machinery for reprocessing. ¢Û Thermal decomposition of waste plastics to produce liquid fuels and chemicals. The basic principle of thermal decomposition technology is to thoroughly decompose the macromolecular chain of the original resin polymer in waste plastic products to return them to a low molecular weight state. Some components are monomers, and other components are basic organic raw materials.
In order to promote the recycling of plastics and prevent environmental pollution, relevant state ministries and commissions have issued relevant regulations, requiring plastic products to be clearly marked with plastic recycling signs. Plastic signs are signs marked on plastic products for recycling, consisting of recycling graphics, plastic codes and corresponding abbreviation codes.
Abbreviation code PET stands for polyester; HDPE stands for high-density polyethylene; PVC stands for polyvinyl chloride; LDPE stands for low-density polyethylene; PP stands for polypropylene; PS stands for polystyrene; OTHER stands for other types of plastic products. By identifying the recycling signs on plastic products, waste plastics can be classified and recycled, thereby saving sorting time, saving manpower and material resources, and improving efficiency.
Mineral water bottles, carbonated beverage bottles, etc., such as Nongfu Spring and Coca-Cola bottles. It is usually heat-resistant up to 70¡ãC, but cannot be filled with hot water, heated, or recycled.
Plastic bags used in supermarkets and shopping malls are mostly made of this material, which can usually withstand high temperatures of 110¡ãC. Plastic bags marked "for food" can be used to hold food.
Containers made of this material are rarely used to package food. This material should be kept away from heat as much as possible. If it encounters high temperatures and grease, the harmful substance single-molecule vinyl chloride is easy to contaminate food.
It is usually used in the production of plastic bottles and films, such as shampoo and hand sanitizer packaging.
Some plastic lunch boxes are usually made of this material, which can be reused after careful cleaning.
This material is usually used to make bowl-shaped instant noodle boxes and foam fast food boxes. It is recommended to avoid storing high-temperature and high-fat foods as much as possible. Under high temperature conditions, this material will decompose polystyrene, which is harmful to the human body.
One or more additives such as stabilizers, plasticizers, colorants, and antioxidants need to be added during plastic processing. Taking plasticizers as an example, the most commonly used are "phthalates", especially DEHP. According to different usage conditions, DEHP can be divided into general grade, food grade and medical grade. The acute toxicity of plasticizers is very low. For example, DEHP can only be lethal to rats if it is taken orally at 30g/kg¡¤bw or more. The main problem of plasticizers is that they produce endocrine disruption effects. For example, in animal experiments, researchers found that DEHP can affect the reproductive development of animals and disrupt endocrine. However, no definite evidence has been found that it has adverse effects on humans, and no corresponding cases have occurred. The International Agency for Research on Cancer currently classifies DEHP as Class 2B (possibly carcinogenic to humans). Plasticizers cannot be used arbitrarily in food packaging materials. National standards have strict regulations on usage, residues or migration.
Plastics have superior properties and are widely used, which has greatly increased their output. In 1996, the world's synthetic resin output reached 129.4 Mt, an increase of 6.7% over 1995. North America, Asia and Europe are the world's main synthetic resin production regions, with output accounting for 32.4%, 28.9% and 28.5% of the world's total output respectively. Compared with 1995, Asia has the largest increase, at 12.3%; North America is second, at 7.8%; Western Europe has the smallest increase, at only 0.5%. The largest synthetic resin producer is the United States, accounting for about 30% of the world's total output, followed by Japan, Germany and South Korea, accounting for 11.4%, 8.4% and 6.0% of the world's total output respectively. Among plastic products, polyethylene is the largest output variety, followed by polyvinyl chloride, polypropylene, polystyrene, etc.
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