Believe that many Chinese friends have met this kind of situation, a dish for packaging in the restaurants, returned home, found that there was a big hole at the bottom of packaging plastic boxes, estimation is the poor quality of plastic boxes are oil of high temperature melt away, also jokingly said that estimates it is half ate a half plastic food, also don’t know what is the material.
This kind of easy hot box is the basic foam box, is made of polystyrene resin as raw material foam. In recent years this kind of box seems to use less in China.
Now there are a variety of materials of the food box, some marked PP, PS, and Pvc materials, that is, do not know what they have what difference, do not know what harm to the human body.
Plastic is an integral part of our life now, single is also can understand from the literal can shape of the material, it is a kind of polymer material, main component is the synthetic polymer compound, also call it synthetic resin, in order to increase the some performance of the product, will add some additives to change its performance, then what is plastic, how to distinguish between all kinds of plastic material.
The earliest plastics appeared in the 19th century. People put camphor and a small amount of alcohol into natural cellulose to make artificial plastics. This kind of material has plasticity. Under hot pressing, various products can be formed, named celluloid. Since then, the history of using plastics has begun. In the 20th century, polymer synthetic plastics have appeared.
In terms of molecular structure, plastic is a kind of polymer, which makes it very easy to shape. It can be understood literally that it is a plastic material. Especially in the high temperature and high pressure state, most of plastic is extracted from fossil fuels. Oil and natural gas are the main raw materials, and they become the perfect materials of plastic.
Plastic manufacturing process
First, crude oil and natural gas are extracted, and then sent to refineries to extract many substances, including the main components of plastics, that is, ethane is extracted from oil, propane is extracted from natural gas, ethane and propane are sent to cracking plants to decompose components, ethane is used to make ethylene, propane is used to make propylene, and then catalyst is added to combine molecules Forming resin polymer, plastic because of this structure is easy to shape under high temperature and pressure.
Polymerization turns ethylene into polyethylene, propylene into polypropylene. These resins are melted and cooled, and then cut into plastic raw material particles. The plastic particles are sent to the factory for heating to make various plastic products.
Most of the plastic raw materials are transparent or milky white. In order to use or design, all kinds of plastic products need to have all kinds of colors, which needs to be dyed in the process of plastic processing. The common dyeing method is to mix a certain proportion of Color Masterbatch or color powder in the raw materials before production, and let the raw materials and color masterbatch or color powder melt together in the injection molding machine to produce the required products Color.
Plastic products of various shapes and sizes are used in life. The main production mode of these plastic products is injection molding. In this process, the three elements needed are plastic raw materials, mold, injection molding machine. The process of injection molding is to put the raw materials into the machine hopper, mix and melt them in the injection molding machine, and then fill the mold with the raw materials. The raw materials are rapidly cooled and molded in the mold After opening the mold, take out the product.
Another common processing method is blow molding. All kinds of plastic bottles, cans, barrels, etc. are blow molding. It is well understood that after the raw materials are melted, blow them up in the mold for molding.
The most intuitive difference between injection molding and blow molding is that the injection product is solid core and blow molding is hollow core.
In our daily life, more than 70% of the daily necessities are plastic products, among which the main plastic materials are PP, PS, PA, PC, PE, ABS, PVC. To distinguish them, we need to understand their characteristics and differences. Before we understand them, we need to understand what is thermosetting plastic and thermoplastic.
Thermosetting plastic is a kind of plastic that needs to be heated to solidify during processing and forming. It is to mix two different kinds of plastic and inject them into the mold at the same time. After the reaction is caused by heating, a single molecule will become a network three-dimensional structure, that is, the plastic that will harden after heating. The advantages are high heat resistance, and the disadvantages are that it is not easy to recycle and reuse. Thermosetting plastic has a FRP bathtub, Tableware and so on.
Thermoplastic is the plastic that will melt after heating, that is, the solid plastic particles will be heated, and then injected into the mold after the plastic melts. After the finished product cools and solidifies, the product will be finished. The advantage is that the product can be recycled, and the disadvantage is that it is not heat-resistant. The thermoplastic includes PE, PP, PS, PVC, ABS, PA, PC, etc.
Each of us has his or her own ID card. It can distinguish everyone’s identity. Can plastic also distinguish? After purchasing plastic products, if you carefully observe them, you will see that there is a number on the shell of the product, which is to distinguish the types of plastic. The plastic identification code developed is universal all over the world. It is a triangle composed of three clockwise rotating arrows, with numbers representing each material marked in the middle. At present, there are seven identification codes, except for We know the plastic material by coding, and the most important thing is to assist in recycling, which can be subdivided and reused.
PET(polyester) code 1, also called polyester resin, the raw material is milky white or light yellow, good transparency, non-toxic, with high density, high hardness, wear-resistant, but not resistant to hot water blister, alkali and other characteristics, the use temperature is 65 ℃ ~ – 20 ℃, more than this temperature easy to release harmful substances, can not be used repeatedly for a long time, 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, is white, non-toxic and tasteless. It has the characteristics of low density, good toughness, acid and alkali resistance, but poor aging resistance. It is difficult to clean and recycle. It is mainly used for washing bottles, wires and cable sheath.
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 and other characteristics. PVC is divided into hard and soft. The difference between them depends on whether soft agent is added. PVC itself is not toxic. If plasticizer and antioxidant are added, it will be toxic. Therefore, soft PVC is not toxic. Hard PVC is toxic To be used in packaging materials, building materials industry in the pipeline, doors and windows.
LDPE (low density polyethylene) code 4, also known as high-pressure polyethylene, the raw material is milky white, tasteless and non-toxic, with good transparency, good softness, alkali resistance, but poor heat resistance and other characteristics, the density is the lowest of all polyethylene, more than 110 ℃ will be released by hot melting toxic substances, mainly used for fresh-keeping film, plastic film, etc.
PP (polypropylene) code 5, also known as 100 fold glue, has small 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 ℃, and can be put into microwave oven for heating, but it is forbidden to use recycled products to contain food. PP materials are commonly used in plastic lunch boxes, water cups, food packaging boxes and other products, because PP has So Australian currency is made of this kind of plastic.
PS (polystyrene) code 6, also called hard rubber, has a higher hardness than PP. The raw material has high transmittance, colorless and tasteless, and easy to dye. However, its toughness is poor and brittle. When the temperature exceeds 70 degrees, it will release harmful substances. It is commonly used in disposable foam lunch boxes, packaging liners, decoration materials, etc., because it is not easy to recycle, and now gradually replace them with paper materials.
PA (polyamide), also known as nylon, has no code. The raw materials are non-toxic and odorless, with good tenacity, wear-resistant, heat-resistant and chemical resistant. There are many kinds of PA plastics. Especially now, by mixing various fiber materials, the performance of PA plastics has been significantly improved, which has replaced some metals, and is widely used in automobile parts, home appliance shell, protective articles and other fields.
ABS plastic also has no code. It is a composite of acrylonitrile (a), butadiene (b) and styrene (c), also known as ABS resin. So ABS has the common performance of these three materials, such as impact resistance, heat resistance, low temperature resistance, good comprehensive performance, low price, wide use, non-toxic, tasteless, translucent or transparent particles in appearance. Many parts of the car are Made of ABS plastic, parts in household appliances and office machines are also made of ABS.
Other categories, code 7, include acrylic, polycarbonate, polylactic acid, etc. the characteristics of these plastics are that they can’t be heated in the microwave oven, can’t be sterilized at high temperature, can’t be directly exposed to the sun, because it may release bisphenol A, cause harm to human body, etc.
As a matter of fact, the development of plastics goes far beyond the above. All kinds of synthetic plastics make people suddenly confused. You may have heard that PBT (together with pet, collectively referred to as thermoplastic polyester), hips (impact resistant polystyrene, commonly known as engineering plastics), POM (polymethylaldehyde thermoplastic crystalline polymer), EVA (copolymerized by ethylene and acetic acid), etc. are too many More, there is no example here.
You will find that there are many plastic products without identification codes, so how can you know what plastic materials they are? In addition to using professional instruments for analysis, there are relatively primary and simple identification methods. Of course, this method is only for reference, and it can not completely determine the type of plastic.
The buoyancy test is to distinguish the material according to the density of the plastic. The density of the common plastic on the market will be between 0.9-2.0g/cm3 when no other components are added. The density of water is exactly 1g / cm3. In the common plastic, only PP and PE have the density less than 1.0, so PP and PE can float on the water surface when they are put into the water. Other plastic materials have high density The water will sink to the bottom.
For combustion test, if you want to distinguish all kinds of plastic materials more clearly, you can distinguish them by burning. Combustion test is to burn all kinds of plastic materials, observe the characteristics and changes in the process, and then judge the plastic materials. The observed states are: the state leaving the fire source, combustion smell, flame color, the state after burning, and the easy burning range The results of burning test are based on the special effects of plastics, which can not be used as a scientific basis to distinguish plastics.
Pet burns and emits black smoke with sour taste.
PP can burn all the time. The flame is yellow and blue. There is a smell of candle. PS can also burn all the time. The flame is yellow and black smoke. After combustion, there is carbon dust and pine oil smell
ABS can burn rapidly and continuously after ignition. The flame is yellow and black smoke. After combustion, it softens without dripping, and the rubber smell is very heavy
The flame of PE is yellow and blue. It softens and drips after burning. There is a smell of paraffin burning. (did you burn plastic bags when you were a child, the feeling of popping and dropping hasn’t been forgotten, remember the pain on your hands?)
PA is not easy to burn, and it will be extinguished after leaving the fire source. The flame is yellow, gray and white smoke. After burning, it bubbles and drips, with the smell of sheepskin and fingernails;
PVC is not easy to burn, and it will go out after leaving the fire source. The flame is blue and green with smoke. It is difficult to soften after burning, and it has an irritating sour taste. (the hard shrink film on the package 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 to make epoxy resin and polycarbonate. It is also widely used as a catalyst in plastic products, such as water bottles, medical devices, milk bottles and other products. Adding bisphenol A can make plastic products colorless and transparent, improve toughness and lightness, It is found through research that bisphenol A is a low toxic chemical, which can cause harm to human body, especially to the health of infants and young children. Now, many countries in China and the world prohibit bisphenol A 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 are turned into garbage, especially some disposable plastic products. A large part of them are not recycled, which will eventually damage animals and the earth environment.
Fortunately, now people have realized its harm, and have begun to take various measures, such as reducing the use of disposable plastics, using reusable substitutes, using some organisms to decompose plastics, and using some microorganisms to shorten the degradation time of 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 At the same time, it can reduce the pollution of plastic to the earth.
Post time: Dec-10-2019