Plastic is an indispensable part of our current life. It can be understood literally as a material that can be shaped. It is a polymer material. The main component is synthetic polymer compounds, also called synthetic resins. For some properties of the product, some additives will be added to change its properties, so what exactly is plastic and how to distinguish various plastic materials.
The earliest plastic appeared in the 19th century. People put camphor and a small amount of alcohol in natural cellulose to make artificial plastic. This material has plasticity and can be molded into various products under hot pressing. With the history of human use of plastics, polymer synthetic plastics began to appear in the 20th century.
In terms of molecular structure, plastic is a polymer, which makes plastic extremely easy to shape. It can also be understood literally as a material that can be shaped, especially under high temperature and high pressure. Most plastics are made from fossils. Extracted from fuel, oil and natural gas are the main raw materials, they become the perfect material for plastics.
plastic manufacturing process
Crude oil and natural gas are first extracted, and then sent to refineries, where many substances are extracted, including the main components of plastics, namely ethane from oil, propane from natural gas, ethane and Propane is sent to a cracking plant to be broken down into molecules, ethane is used to make ethylene, and propane is used to make propylene, and then a catalyst is added to combine the molecules to form resinous polymers. Plastics are easily shaped at high temperatures and pressures because of this structure.
Polymerization turns ethylene into polyethylene and propylene into polypropylene. These resins are melted and cooled, and then cut into plastic raw material pellets. The plastic pellets are sent to factories to be heated to make various plastic products.
Most of the plastic raw materials are transparent or milky white. Various plastic products are required to have various colors for the needs of use or design, which requires dyeing during the plastic processing; the commonly used dyeing method is to add in the raw materials before production. Mix a certain proportion of color masterbatch or toner, and let the raw material and color masterbatch or toner melt together in the injection molding machine to produce the desired product color.
Plastic products of various shapes and sizes are used in life. The main production method of these plastic products is injection molding. The three elements required in this process are plastic raw materials, molds, and injection molding machines. The process of injection molding is to put the raw materials into Put the raw materials into the hopper of the machine, after mixing and melting in the injection molding machine, the raw materials are filled into the mold, the raw materials are rapidly cooled in the mold, and finally the mold is opened to take out the product.
Another common processing method is blow molding. All kinds of plastic bottles, cans, barrels, etc. are all blow molding. It is well understood that after the raw materials are melted, they are blown up in the mold.
The most intuitive difference between injection molding and blow molding is that injection molding products are solid core bodies, while blow molding products are hollow core bodies.
In daily life, more than 70% of the daily necessities are plastic products. The main plastic materials are PP, PS, PA, PC, PE, ABS, PVC. To distinguish them, you must know their Features and differences, before understanding them, we must first understand what thermosets and thermoplastics are.
Thermosetting plastics are plastics that need to be heated to solidify during processing, that is, two different plastics are mixed and injected into the mold at the same time. Hardened plastic has the advantage of high heat resistance, but the disadvantage is that it is not easy to recycle and reuse. Thermosetting plastics include bathtubs and tableware made of FRP.
Thermoplastics are plastics that will melt after heating, that is, heating solid plastic particles, and then injecting them into the mold after the plastic is melted. After the finished product is cooled and solidified, the product is completed. The advantage is that the product can be recycled. The disadvantage is that it is not Heat-resistant, thermoplastics include PE, PP, PS, PVC, ABS, PA, PC, etc.
Each of us has our own ID card, which can distinguish everyone’s identity. Can plastic also identify? After buying plastic products, if you look carefully, you will see that there are numbers on the outer shell of the products. This is to distinguish the types of plastics, and developed into a plastic identification code. This identification code is universal all over the world. It is composed of A triangle formed by three clockwise rotating arrows is marked with a number representing each material in the middle. There are currently 7 identification codes. These identification codes not only allow us to understand plastic materials through coding, but the most important thing is to assist recycling, which can subdivide the types and Reuse.
PET (polyester) code 1, also known as polyester resin, the raw material is milky white or light yellow, with good transparency, non-toxic, high density, high hardness, wear resistance, but not hot water soaking, alkali resistance, etc. Features: The operating temperature is 65°C~-20°C. If the temperature exceeds this temperature, it is easy to release harmful substances and cannot be used repeatedly for a long time. It is mainly used for beverage bottles in packaging materials, mineral water bottles, shells and accessories in electronic appliances, etc.
HDPE (high density polyethylene) code 2, also known as low pressure ethylene, the raw material is white, non-toxic and tasteless, with low density, good toughness, acid and alkali resistance, but poor anti-aging performance, etc. It is difficult to clean and do not recycle. Used in detergent packaging bottles, wire and cable jackets, etc.
PVC (polyvinyl chloride) code 3, also known as adhesive film, the raw material is yellow and translucent, with good transparency, high strength, good flexibility, not easy to be brittle, etc. PVC has soft and hard points, the difference between them depends on Whether a softener is added or not, PVC itself is not toxic, if it is added with plasticizers, antioxidants, etc., it is toxic, so soft PVC is not toxic, rigid PVC is toxic, mainly used in packaging materials, pipes in the building materials industry , doors and windows, etc.
LDPE (low density polyethylene) code 4, also known as high pressure polyethylene, the raw material is milky white, odorless and non-toxic, with good transparency, good flexibility, alkali resistance, but poor heat resistance, etc. The density is all polyethylene. The lowest in ethylene, it will melt and release toxic substances when it exceeds 110 ℃, mainly used for cling film, plastic film, etc.
PP (polypropylene) code 5, also known as Baizhe glue, has low density, transparent and light appearance of raw materials, non-toxic and tasteless, high strength and elasticity, chemical resistance, collision resistance, high temperature resistance of 100~120 degrees Celsius, can be put It is heated in a microwave oven, but it is forbidden to use recycled products to hold food. It is often used in plastic lunch boxes, water cups, food packaging boxes and other products. PP materials are used. Because of these characteristics of PP, Australia’s currency is made of this plastic. .
PS (polystyrene) code 6, also known as hard glue, the hardness will be higher than PP, the raw material has high light transmittance, colorless and odorless, easy to dye, but poor toughness, easy to brittle, the temperature exceeds 70 ℃ will release harmful Substances are commonly used in disposable foam lunch boxes, packaging linings, decoration materials, etc. Because it is not easy to recycle, they are gradually replaced by paper materials.
PA (polyamide) is also called nylon. It has no code. The raw material is non-toxic and odorless, with good toughness, wear resistance, heat resistance and chemical resistance. There are many kinds of PA plastics. Especially now, by mixing various fiber materials, PA The various properties of plastics have been significantly improved, and they have replaced some metals, and are widely used in auto parts, household appliance housings, protective equipment and other fields.
ABS plastic has no code. It is a composite of three substances: acrylonitrile (A), butadiene (B), and styrene (C). It is also called ABS resin, so ABS also has the common properties of these three substances. Impact resistance, heat resistance, low temperature resistance, good comprehensive performance, cheap price, wide range of uses, ABS plastic is non-toxic, tasteless, and the appearance is translucent or transparent particles, many parts in automobiles are made of ABS plastic, in household appliances , The parts on the office machine are also made of ABS.
Other types, code 7, include acrylic, polycarbonate, polylactic acid, etc. The characteristics of these plastics are that they cannot be heated in a microwave oven, cannot be sterilized at high temperature, and cannot be exposed to direct sunlight, because this may release bisphenol A, which will cause harm to the human body Wait.
In fact, the development of plastics is far more than the above. Various synthetic plastics make people instantly confused. You may have heard, PBT (collectively referred to as thermoplastic polyester together with PET), HIPS (impact-resistant polyphenylene) Ethylene, which is often referred to as engineering plastics), POM (polyoxymethylene thermoplastic crystalline polymer), EVA (copolymerized between ethylene and acetic acid), etc., etc., there are too many, and I will not give examples here.
You will find that there are still many plastic products without identification codes, so how can you know what kind of plastic they are? In addition to using professional instruments for analysis, there are relatively simple identification methods. Of course, this method is only a reference and cannot completely determine the type of plastic.
The buoyancy test is to distinguish the material of the plastic according to its density. At present, the density of common plastics on the market will be between 0.9~2.0g/cm³ when no other ingredients are added, and the density of water is exactly 1g/cm³. Among the common plastics, only PP and PE have a density of less than 1.0, so PP and PE are the ones that can float on the water surface when placed in water. Other plastic materials will sink to the bottom because their density is greater than that of water.
Burning test, if you want to distinguish various plastic materials more clearly, you can distinguish it by burning. The burning test is to burn various plastics, observe the characteristics and changes in the process, and then judge the plastic. Material, the observed states are, the state of leaving the fire source, the burning smell, the color of the flame, the state after burning, the degree of flammability, and the burning test is based on the special effects of plastics, and cannot be used as a science to identify plastics in accordance with.
PET burns with black smoke and has a sour taste
PP can burn all the time, the flame is yellow on top and blue on the bottom, there is a smell of candles
PS can also burn all the time, the flame is yellow black smoke, there is carbon powder after burning, and there is a smell of pine oil
ABS can burn quickly and continuously after being ignited, the flame is yellow black smoke, softened without dripping after burning, and the smell of rubber is very heavy
PC is more difficult to burn, and it will go out when the fire is taken away. The flame is a small amount of black smoke, which softens and foams after burning, and has no special smell;
The PE flame is yellow on the top and blue on the bottom. After burning, it softens and drips, and there is a burning smell of paraffin; (Have you ever burned a plastic bag when you were a child? You haven’t forgotten the feeling of dripping, do you remember the pain when you drip it on your hand? ?)
PA is not easy to burn, it will go out when it leaves the fire source, the flame is yellow gray and white smoke, foaming and dripping after burning, with the smell of sheepskin and nails;
PVC is not easy to burn, it goes out when it leaves the fire source, the flame is blue on top and green on the bottom with smoke, it is difficult to soften after burning, and has a pungent sour taste; (the shrink film on the packaging, the hard one is PCV)
As people pay more and more attention to physical health, bisphenol A often appears in various reports. Bisphenol A, also known as BPA, white crystal, is mainly used in the manufacture of epoxy resin and polycarbonate, and is also used as a Catalysts are widely used in plastic products, such as water bottles, medical equipment, milk bottles and other products. The addition of bisphenol A can make plastic products colorless and transparent, improve toughness, lightness, durability and other characteristics. After research, it was found that bisphenol A is a It is a kind of low-toxicity chemical, which can cause harm to the human body, especially the health of infants and young children. Now China and many countries in the world have stipulated that bisphenol A is prohibited from being used in baby bottles, water bottles and other products.
Because of the increasing use of plastics, the global plastic production is also increasing. Most of these plastics eventually become garbage, especially some single-use plastic products. A large part of them are not recycled and will eventually harm animals. , destroy the earth’s environment.
Fortunately, people have now recognized its hazards and have begun to take various measures, such as reducing the use of single-use plastics, using reusable alternatives, using some organisms to decompose plastics, and using some microorganisms to degrade plastics. From hundreds of years to just a few days, plastics are almost everywhere. Only by fully understanding plastics can we make good use of plastics and reduce the pollution of plastics to the earth.
In fact, people often say to protect the environment and protect the earth. My understanding is that the earth is 4.6 billion years old. What kind of strong winds and waves have not experienced, do we still need human protection? Whether it is plastic, garbage, or human beings, they are all part of the earth. No matter how much you toss or pollute, it is like a little dermatitis on the surface of the earth. Once you apply the ointment, the virus and fungi are gone, and the earth is still the earth! Therefore, to manage the environment and reduce pollution is not to save the earth, but to save human beings.