I think my friends have come across this situation. I packed a meal at a restaurant and when I got home I found a big hole in bottom of a packed plastic lunch box. The quality of plastic lunch box was estimated to be too poor and greasy. The heat was melting, and he joked that he probably ate half plastic and half food for this meal, and he did not know what material it was made of.
This type of packaging box, which is easy to scald, is mainly a foam box, which is made of polystyrene resin as raw material. Especially for takeaway food, they all use transparent boxes.
While reviewing relevant information, editor also discovered an interesting situation: in 1999, it was stipulated that foam cookware should be completely phased out by end of 2000, meaning that it could not be used or produced. White pollution is too serious, and ban was lifted on February 26, 2013, so although it is used less now, it is still used all time.
Now there are lunch boxes in different materials, some with PP label, some with PS, and some with PVC material. I don't know how they differ, and I don't know which ones are harmful to humans. body.
Plastic is now an integral part of our lives. It can be understood literally as a material that can be shaped. This is a polymer material. The main component is a synthetic polymer compound, also called synthetic resin. will increase some of characteristics of product, some additives will be added to change its characteristics, so what is plastic and how to distinguish between different plastic materials.
The very first plastics appeared in 19th century. For manufacture of artificial plastics, camphor and a small amount of alcohol were added to natural cellulose. Since then, history of human use of plastics began, and polymer synthetic plastics began to appear in 20th century.
In terms of molecular structure, plastic is a type of polymer that makes plastic very easy to shape. It can also be taken literally as a material that can be shaped, especially at high temperature and high pressure. Extracted from fossil fuels, oil and gas, being main raw material, become ideal materials for plastics.
Image Source: pixabayPlastic production process
Crude oil and natural gas are first extracted and then sent to refineries to extract many types of substances, including main components of plastics, that is, ethane is extracted from oil, propane, ethane and propane are extracted from natural gas. 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 molecules to form a polymer resin. easily deformed under high temperature and pressure.
Polymerization turns ethylene into polyethylene and propylene into polypropylene. These resins are melted and cooled, and then cut into pellets of plastic raw materials. The plastic pellets are sent to factory to be heated to produce various plastic products.
Image source: pixabay
Most plastic raw materials are transparent or milky white. Different plastic products require different colors for use or design needs, which requires dyeing during plastic processing; most commonly used staining method is pre-staining. Mix a certain proportion of a color masterbatch or color. powder in raw material, and let raw material and color masterbatch or color powder melt together in an injection molding machine to obtain the desired product color.
Image source: pixabay
Plastic products of various shapes and sizes are used in everyday life. The main production method for these plastic products is injection molding. Three elements are needed in this process: plastic raw materials, molds, and injection molding machines. Injection Molding Process Put raw material into hopper of machine, stir and melt it in injection molding machine, and then pour raw material into mold. The raw material is quickly cooled and molded into mold, and finally mold is opened to take out product.
Another common processing method is blow molding. All kinds of plastic bottles, jars, barrels, etc. are blow molding. It is common knowledge that after raw materials are melted, they are blown into a mold for molding.
The most intuitive difference between injection molding and blow molding is that injection molding products are solid cores while blow molding products are hollow.
In everyday life, more than 70% of essential items are made of plastic. The main plastic materials are PP, PS, PA, PC, PE, ABS, PVC, etc. In order to distinguish them, it is necessary to understand their characteristics. and differences, before understanding them, we must first understand what thermoset plastics and thermoplastics are.
Thermoset plastics are plastics that must be heated to solidify during processing and molding. Two different plastics are mixed and injected into mold at same time. After heat-induced reaction, one molecule becomes a three-dimensional network structure, that is, plastic that solidifies after heating has advantage of high temperature resistance, but disadvantage is that it is not easy to recycle and reuse. tubs and utensils made of fiberglass.
Thermoplastic is a plastic that melts after heating, that is, solid plastic particles are heated and then injected into a mold after plastic is melted. After finished product is cooled and solidified, product is completed Advantages The product can be recycled, but disadvantage is that it is not heat resistant Thermoplastics include PE, PP, PS, PVC, ABS, PA, PC and so on.
We each have our own ID that can identify everyone, so can plastic also identify them? If you look closely at a plastic product after purchase, you will see that there is a number on body of product. This is a plastic identification code designed to distinguish type of plastic. This identification code is common throughout world. It consists of a triangle composed of three clockwise rotating arrows, with numbers representing each material marked in middle, currently has 7 identification codes, these identification codes not only allow us to understand plastic materials through codes, but, most importantly, promote recycling, which can subdivide types and reuse.
PET (polyester) code name 1, also known as polyester resin, milky white or light yellow raw material, good transparency, non-toxic, high density, high hardness, wear resistance, but not it is resistant to hot water soaking, not resistant to alkali, etc. The use temperature is 65 ° C ~ -20 ° C. If temperature exceeds this temperature, harmful substances will be easily released, and it cannot be reused for a long time. It is mainly used in beverage bottles, mineral water bottles and electronic appliances in packaging materials. In shell and accessories.
HDPE (high density polyethylene), code name 2, also known as low pressure ethylene, raw material is white, non-toxic and tasteless, with low density, good toughness, acid and alkali. resistance, but poor anti-aging, etc. Features that are difficult to clean and not recycle, are mainly used for detergent bottle packaging, wire and cable sheathing, etc.
PVC (polyvinyl chloride) code name 3, also known as adhesive film, raw material is yellow and translucent, with good transparency, high strength, good flexibility, not brittle, etc. PVC has soft and hard The difference between them depends on whether a softener is added. PVC itself is non-toxic. If plasticizers, anti-aging agents, etc. are added, it will be toxic, so soft PVC is non-toxic, and hard PVC is toxic. Used in packaging materials, pipes, doors and windows in building materials industry, etc.
LDPE (low density polyethylene), code name 4, also called high pressure polyethylene, raw material is milky white, odorless and non-toxic, with good transparency, good flexibility, alkali resistance, but heat resistance Poor heat resistance and other characteristics, density is lowest among all polyethylenes, and it releases toxic substances when it exceeds 110 ° C. Mainly used for food film, plastic film, etc.
PP (polypropylene) code name 5, also known as corrugated adhesive, has low density, transparent and light appearance, non-toxic and tasteless, high strength and elasticity, chemical resistance and impact resistance . resistance, resistance to high temperatures of 100 ~ 120 degrees Celsius, can be heated in microwave, but it is forbidden to use recycled food storage products, commonly used in plastic lunch boxes, water glasses, food packaging boxes and other products. made of polypropylene materials, because of these characteristics of polypropylene, so currency of Australia is made of this plastic.
PS (polystirol) code name 6, also known as hard glue, has a higher hardness than PP, high light transmittance, colorless and odorless, easy to color, but low strength, easy to color. brittle and release harmful substances at temperatures above 70 °C. It is often used in disposable foam lunch boxes, packaging liners and finishing materials. Since it is not easy to recycle, it is gradually being replaced by paper materials.
PA (polyamide) also called nylon, no code name, raw material non-toxic and odorless, good toughness, wear resistance, heat resistance, chemical resistance, there are many varieties of PA plastics, especially now, by mixing different fiber materials, various properties of PA plastics have been greatly improved, and some metals have been replaced, and they are widely used in auto parts, home appliance housings, security equipment and other fields.
ABS has no code name, it is a compound of acrylonitrile (A), butadiene (B) and styrene (C), also called ABS resin, so ABS also has common properties. of these three substances, impact resistance, heat resistance, low temperature resistance, good overall performance, cheap price, wide range of applications, ABS plastic is non-toxic, tasteless, and appearance is translucent or transparent particles. It is made of ABS plastic, and parts of household appliances and office equipment are also made of ABS.
Other categories codenamed 7, including acrylic, polycarbonate, polylactic acid, etc. The characteristics of these plastics are that they cannot be heated in a microwave oven, sterilized at high temperature, and cannot be directly exposed to sun, because they can release bisphenol A, which is harmful to human body. They will cause harm, etc.
In fact, development of plastics goes far beyond above. All kinds of synthetic plastics confuse people. You may have heard that PBT (collectively referred to as thermoplastic polyester along with PET), HIPS (high impact polyester)-resistant polystyrene often referred to as engineering plastics, POM (polyoxymethylene thermoplastic crystalline polymer), EVA (ethylene-acetic acid copolymer), and etc., etc., too much, not shown here with an example.
You'll find that many plastic items still don't have identification codes, so how do you know what plastic they're made from? In addition to using professional instruments for analysis, there is also a relatively elementary and simple identification method, of course, this method is only a reference and cannot fully determine type of plastic.
Buoyancy test allows you to distinguish material by density of plastic. At present, when no other ingredients are added to conventional plastics on market, density will be between 0.9 and 2.0. g/cm³. The density of water is exactly 1 g/cm³. Among common plastics, only PP and PE have a density less than 1.0, so PP and PE can float on water when it enters water. Other plastics sink to bottom because their density is higher than that of water.
Combustion test. If you want to distinguish between different plastics more clearly, you can tell them apart by burning them. The combustion test consists of burning various plastics and observing what is produced in process. The characteristics and changes of plastic, and then judge material of plastic. Observed states: exit state of fire, smell of burning, color of flame, state after burning, and degree of flammability. The combustion test is entirely based on special effects of plastic. The conclusion cannot be used as scientific basis for plastic identification.
PET burns with black smoke and tastes sour
PP can burn constantly, flame is yellow at top, blue at bottom, with a candle smell
PS can also burn constantly, flame is yellow and black smoke, after burning coal dust and smell of coniferous oil
ABS plastic, once ignited, can burn quickly and continuously. Yellow flame with black smoke. After burning, it softens without dripping and has a strong smell of rubber.
PC is harder to burn and will go out when fire is removed. The flame is a small amount of black smoke that softens and foams after burning, and has no particular smell;
The polyethylene flame is yellow at top and blue at bottom. After burning, it softens and drips, has smell of burning paraffin; remember?)
PA is not easy to burn and will go out when it leaves source of fire. The flame is a yellow-gray smoke. After burning, it foams and drips, has a smell of sheepskin and nails;
PVC is not easy to burn and will go out when it leaves source of fire. The flame is blue on top and green on bottom with smoke. Once burned, it is difficult to soften and has a sharp, sour taste. smell; (shrink film on packaging, rigid - PVC)
As people pay more and more attention to their health, bisphenol A is often mentioned in various reports. Bisphenol A, also known as BPA, is a white crystal, mainly used in production of epoxy resin and polycarbonate. As a catalyst, it is widely used in plastic products such as water bottles, medical equipment, feeding bottles, etc. The addition of BPA can make plastic products colorless and transparent, improve strength, lightness, durability and other characteristics. After research, it was found that bisphenol A is a low-toxic chemical, and frequent exposure can harm human body, especially health of babies. Now, China and many countries in world have banned use of bisphenol A in baby feeding bottles, water bottles and other products.
Due to growth in use of plastic, global production of plastic is also increasing. Much of this plastic ends up in trash, especially some single-use plastic items. Most of them are not recycled. It will harm animals and destroy earth's environment.
Image source: pixabay
Fortunately, people have now realized its harm and have begun to take various measures such as reducing use of single-use plastic, introducing alternatives that can be reused, using some organisms to degrade plastic, and using some microorganisms to degrade plastic. time it takes for plastics to decompose has been reduced from hundreds of years to days. Plastics are almost everywhere. Only by fully understanding what plastics are, can we use plastics effectively and reduce plastic pollution on ground.
In fact, people often talk about protecting environment and earth. My understanding is that Earth has been around for 4.6 billion years and has never had a major storm. Does she need human protection? Whether it's plastic, trash or people, they are all part of earth. No matter how much you throw and pollute, it's like a little dermatitis on surface of earth. Once ointment is applied, it's all over, virus and fungus will disappear, and earth will remain earth! Thus, managing environment and reducing pollution should not save earth, but people.
More articles about machines, production, principles and haberdashery can be found in our public WeChat account "Production Principles".