Production of plastic bags:
Mainly use "Ziegler-Natta" catalyst to produce polyaddition reaction of olefins into polymers. PE (polyethylene) or PVC (polyvinyl chloride) plastic particles are heated and melted into a viscous fluid. They enter the hopper and are extruded through the narrow gap of the screw extruder. In order to prevent deformation, they should immediately enter the coolant (usually water) It is cooled and formed, and rolled into "plastic paper" one by one using a traction roller. There is also a method of calendering, but there is no "polyplastic" because it is not a very thick plastic product and does not require a mold. Break the large plastic paper into small pieces and seal one end with a heat sealer to make a plastic tape.
Polyethylene (PE), referred to as PE. It is a polymer organic compound formed by the addition polymerization of ethylene. Polyethylene is recognized around the world as the best material for food contact. It is non-toxic, tasteless and odorless, and complies with food packaging hygiene standards. Polyethylene film is light and transparent, with properties such as moisture-proof, anti-oxidation, acid-proof, alkali-resistant, average air tightness, and excellent heat sealability. It is known as "plastic flowers". It is the most widely used and most important material for plastic packaging printing.
PVC - polyvinyl chloride
Polyvinyl chloride, referred to as PVC, has a molecular formula of (CH—CH)n. It is the most important type of vinyl polymer and is currently the world’s The second largest plastic variety in the world after polyethylene. Domestic production ranks first among plastics. In the printing industry, polyvinyl chloride color calendered film and transparent glazing film are often used to make the packaging and decoration of covers of books, folders, tickets, etc.
Shrinkfilm - Shrink film
Shrinkfilm is a film that is stretched and oriented during the production process. It will shrink when it is treated with hot air or irradiated with infrared rays during use. Shrunk thermoplastic film. After heat treatment, the film is tightly wrapped on the packaged object, the shrinkage force reaches its maximum value during the cooling stage, and can be stored for a long time.
LDPE - low-density polyethylene
Low-density polyethylene, referred to as LDPE, is the most widely used variety in the plastic packaging and printing industry in various countries. The specific gravity is 0.92~0.93 and can float in water. The crystallinity is low (60), and bulk polymerization is performed at a pressure of 1000 to 3000kg/C㎡, so it is also called high-pressure polyethylene. At 23 degrees Celsius, the density is around 0.92. It has poor calendering formability and is suitable for processing into tubular films by blow molding. Suitable for food packaging, fiber product packaging, and daily chemical product packaging.
HDPE - high-density polyethylene
High-density polyethylene, referred to as HDPE. The density is 0.941~0.965g/cubic centimeter. It is formed by low-pressure polymerization, so it is also called low-pressure polyethylene. It is milky white, indicating poor gloss. The film can be processed by blow molding and T-die extrusion. It is heat-resistant, retort-resistant, cold-resistant and freeze-resistant, has good moisture-proof, gas-proof, and insulating properties, is not easily damaged, and is twice as strong as LDPE. Very easy to speak. It has strong paper-like properties and is known as "paper-like plastic film".
BOPP--biaxially oriented polypropylene film
Biaxially oriented polypropylene film, also known as biaxially oriented polypropylene film, English abbreviation code BOPP, is characterized by stretch The stretching molecules are oriented, and the mechanical strength, folding strength, air density, and moisture-proof barrier properties are better than ordinary plastic films. Compared with cellophane based on unit area, the unit price is lower than cellophane. Because this kind of film has excellent transparency, the color reproduction after inner printing is particularly bright and beautiful, and it is an important base material for plastic composite flexible packaging.
Identification of the toxicity of plastic bags
Commonly used food plastic bags are mostly made of polyethylene film. The film is non-toxic and can be used to hold food. There is also a film made of polyvinyl chloride. PVC itself is non-toxic, but the additives added according to the purpose of the film are often substances harmful to the human body and have a certain degree of toxicity. Therefore, this type of film and plastic bags made of this film are not suitable for holding food. If you want to identify polyvinyl chloride plastic bags and polyethylene plastic bags, you can use the following simple method to identify them.
Polyethylene film (non-toxic)
The film is milky white and translucent (especially obvious when several layers are stacked), and feels smooth, as if there is a wax layer on the surface. Shake it hard, the sound will be brittle, it will be flammable when exposed to fire, the flame will be yellow, mucus will drip when burning, and there will be the smell of burning candles.
Polyvinyl chloride film (generally toxic)
If no pigment is added, it will be transparent and the surface will feel sticky. If shaken hard, the sound will be low and it will not burn easily in case of fire. , it goes out as soon as it is away from the flame, and the flame turns green.
The history of plastic bags: A photographer's experiments in the darkroom led to the creation of the original plastic. Alexander Parks had many hobbies, photography being one of them. In the 19th century, people were not able to buy ready-made photographic film and chemicals like they are today, and had to often make what they needed themselves. So every photographer must also be a chemist.
One of the materials used in photography is "collodion", which is a solution of "cellulose nitrate", a solution of cellulose nitrate in alcohol and ether. It was used to adhere light-sensitive chemicals to glass to create the equivalent of today's photographic film.
In the 1850s, Parks examined different methods of processing collodion. One day, he tried mixing collodion with camphor. To his surprise, the mixture produced a hard material that was bendable. Parks called the substance "paxine," and it was the earliest plastic.
Parks used "Paxin" to create a variety of items: combs, pens, buttons and jewelry-printed jewelry. However, Parks had little business sense and lost money on his business ventures. In the 20th century, people began to discover new uses for plastics. Almost everything in the home can be made from some form of plastic.
It was left to other inventors to continue to develop and profit from Parks's work. John Wesley Hyatt, a printer from New York, saw the opportunity in 1868, when a company that made billiard balls complained about a shortage of ivory. Hayat improved the manufacturing process and gave "Paxin" a new name - "Celluloid" (fake ivory - translator's annotation). He found a ready market from billiard ball manufacturers and was soon making a variety of products out of plastic.
Early plastics caught fire easily, which limited the scope of products made from it. The first plastic that could successfully withstand high temperatures was "Bakerite" (ie, phenol-coated plastic - translator's annotation). Leo Backlund received the patent in 1909.
In 1909, Baekeland in the United States synthesized phenolic plastic for the first time. In the 1930s, nylon came out again. It was said to be "a fiber synthesized from coal, air and water, thinner than spider silk, harder than steel, and better than silk." Their emergence laid the foundation for the invention and production of various plastics thereafter. Due to the development of the petrochemical industry during World War II, petroleum replaced coal as the raw material for plastics, and the plastics manufacturing industry also developed rapidly.
Plastic is a very light substance that can be softened by heating at a very low temperature and can be made into various shapes as desired. Plastic products are brightly colored, light in weight, not afraid of falling, economical and durable. Its advent not only brings a lot of convenience to people's lives, but also greatly promotes the development of industry.
However, the invention of plastics was less than 100 years ago. If people were ecstatic about their birth at that time, now they have to deal with these plastics that have filled our lives and brought great harm to the human living environment. Threatening things and painstakingly.
Plastics are chemical petroleum products extracted from petroleum or coal. Once produced, they are difficult to degrade naturally. Plastics will not decay and degrade even if they remain underground for 200 years. A large amount of plastic waste buried underground will destroy the permeability of the soil, cause the soil to harden, and affect the growth of plants. If domestic animals accidentally eat plastic mixed into feed or left in the wild, they can also die due to digestive tract obstruction.
At present, China's annual plastic output is 3 million tons, and its consumption is more than 6 million tons.
The annual output of plastics in the world is 100 million tons. If calculated based on 15% of the annual plastic waste, the annual plastic waste in the world is 15 million tons. The annual plastic waste in China is more than 1 million tons. The proportion of discarded plastics in the garbage The proportion accounts for 40%. Such a large amount of waste plastics are buried underground as garbage, which will undoubtedly bring greater pressure to the already scarce arable land.
While plastics bring convenience to people's lives, they also bring troubles to the environment that are difficult to deal with. People call the disasters plastics bring to the environment "white pollution."
Currently, many countries use incineration (thermal energy regeneration) or reprocessing (product regeneration) to deal with waste plastics. These two methods enable waste plastics to be reused and achieve the purpose of saving resources. However, since waste plastics will produce gases harmful to the human body and pollute the environment when they are incinerated or reprocessed, it can be said that the disposal of waste plastics is still a major headache in environmental protection work.