Ziegler-Natta catalyst is mainly used for addition polymerization of olefins into polymers. PE (polyethylene) or PVC (polyvinyl chloride) plastic particles are heated and melted into viscous fluid, which enters the hopper and is extruded through the narrow gap of the screw extruder. In order to prevent deformation, they should be immediately cooled in cooling liquid (usually water) and rolled into "plastic paper" with traction rollers. There is also calendering, but there is no "poly molding" because it is not a very thick plastic product and does not need a mold. Break the large plastic paper into small pieces and seal one end with a heat sealing machine to form a plastic belt.
Polyethylene (PE) for short. It is a high molecular organic compound formed by addition polymerization of ethylene. Polyethylene is recognized as the best material in contact with food in the world. Non-toxic, tasteless, odorless, in line with food packaging hygiene standards. Polyethylene film is light and transparent, with moisture resistance, oxygen resistance, acid resistance, alkali resistance, general air tightness and excellent heat sealing performance. Known as "plastic flower making". It is the most commonly used and important material for plastic packaging and printing.
PVC-polyvinyl chloride
The molecular formula of polyvinyl chloride (PVC) is (ch-ch) n, which is the most important vinyl polymer and the second largest plastic variety in the world after polyethylene. In China, the output ranks first among plastics. In the printing industry, PVC color calendering film and transparent glazing film are often used to make packaging and decoration of books, folders, tickets and other covers.
Shrinkage film
Shrinkfilm is a kind of thermoplastic film which is stretched and oriented in the production process and will shrink when it is treated with hot air or irradiated by infrared rays in the use process. After heat treatment, the film is tightly wrapped on the package, and the shrinkage force reaches the maximum in the cooling stage, which can be stored for a long time.
LDPE-low density polyethylene
Low density polyethylene (LDPE) is the most widely used variety in plastic packaging and printing industry in various countries. The specific gravity is 0.92 ~ 0.93, and it can float in water. The crystallinity is low (60%), and bulk polymerization is carried out under the pressure of 1000 ~ 3000 kg/cm ㎡, so it is also called high pressure polyethylene. At 23 degrees Celsius, the density is about 0.92. The calendering formability is poor, and it is suitable to be processed into tubular films by blow molding. Suitable for food packaging, fiber products packaging and daily chemical products packaging.
HDPE-high density polyethylene
High density polyethylene, HDPE for short. The density is 0.941~ 0.965g/cm3. It is made 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 process. Heat and cooking resistance, cold and frost resistance, moisture-proof, gas-proof, good isolation performance, not easy to break, and its strength is twice that of LDPE. It is easy for people to talk about strong paper, which is called "paper-like plastic film".
BOPP biaxially oriented polypropylene
Biaxially oriented polypropylene, also known as biaxially oriented polypropylene, is abbreviated as BOPP, which is characterized by the orientation of stretched molecules, and its mechanical strength, folding strength, air density and moisture-proof barrier are superior to ordinary plastic films. Compared with cellophane per unit area, the unit price is lower than cellophane. Because this film has excellent transparency, the reproduced color after internal printing is particularly bright and beautiful, and it is an important base material of plastic composite soft packaging.
Toxicity identification of plastic bags
Commonly used food plastic bags are mostly polyethylene films, which are non-toxic and can be used to hold food. There is also a membrane made of PVC, which is not toxic in itself, but the additives added according to the purpose of the membrane are often harmful to the human body and have certain toxicity. Therefore, this film and plastic bags made of this film are not suitable for holding food. If you want to identify PVC plastic bags and polyethylene plastic bags, you can use the following simple methods to identify them.
Polyethylene film (nontoxic)
The film is milky white, translucent (especially obvious after being folded several times), smooth to the touch, as if the surface is coated with a layer of wax, which is hard to shake, brittle in sound, flammable in case of fire, yellow in flame, dripping with mucus when burning, and smelling of candles when burning.
PVC film (usually toxic)
If no pigment is added, it is transparent, the surface feels a little sticky, shaking badly and the sound is low. It is not easy to burn in case of fire, and it goes out immediately after leaving the flame, and the flame is green.
History of plastic bags: A photographer's experiment in a darkroom led to the first plastic. Alexander Parks has many hobbies, and photography is one of them. In the19th century, people can't buy ready-made photographic films and chemicals as they do today. They must always make what they need. So every photographer must also be a chemist.
One of the materials used in photography is "collodion", which is a kind of "nitrocellulose" solution, that is, nitrocellulose solution in alcohol and ether. At that time, it was used to stick photosensitive chemicals on glass and make photographic film equivalent to today.
In the 1950s of 19, Parks studied different methods to treat collodion. One day, he tried to mix collodion with camphor. To his surprise, a flexible hard material was produced after mixing. Parks called this substance "Paxsin", which was the earliest plastic.
Parks made all kinds of objects with Paxsin: combs, pens, buttons and jewelry patterns. However, Parks is not very business-conscious. He lost money in his business venture. In the 20th century, people began to explore new uses of plastics. Almost everything in the house can be made of some kind of plastic.
Leave it to other inventors to continue to develop Parks' achievements and profit from them. John wesley Heath, a printer from New York State, saw this opportunity in 1868, when a billiards company complained about the shortage of ivory. Heath improved the manufacturing process and gave Paxsin a new name-celluloid. He got a ready-made market from billiards manufacturers, and soon he made various products out of plastic.
Early plastic caught fire easily, which limited the range of products made from it. The first plastic that can successfully resist high temperature is "Beckett". Leo Baekeland obtained this patent on 1909.
1909, phenolic plastics were synthesized for the first time in Baekeland, USA. In 1930s, nylon came out again, which was called "a kind of fiber composed of coal, air and water, thinner than spider silk, harder than steel and better than silk". Their appearance laid the foundation for the invention and production of various plastics. Due to the development of petrochemical industry in World War II, the raw material of plastics replaced coal with oil, and the plastic manufacturing industry also developed rapidly.
Plastic is a very light substance, which can be softened by heating at a very low temperature and made into various shapes at will. Plastic products are bright in color, light in weight, not afraid of falling, economical and durable. Its appearance not only brings a lot of convenience to people's lives, but also greatly promotes the development of industry.
However, plastics were invented less than 100 years ago. If people were ecstatic about their birth at that time, now they have to deal with these things that are full of life and pose a great threat to the living environment of mankind.
Plastic is a chemical petroleum product extracted from petroleum or coal, and it is difficult to degrade naturally once it is produced. Plastics will not rot and degrade after 200 years underground, and a large amount of plastic garbage will destroy the permeability of soil, harden the soil and affect the growth of plants. If domestic animals eat plastic mixed with feed or left in the wild by mistake, they will also die because of digestive tract obstruction.
At present, the annual output of plastics in China is 300,000 tons and the consumption is more than 6 million tons. The annual output of plastics in the world is 654.38 billion tons. If the annual plastic waste is calculated as 654.38+05%, the annual plastic waste in the world is 6.5438+05 million tons, and the annual plastic waste in China is more than 6.5438+00 million tons, accounting for 40% of the waste. So a lot of waste plastics are buried underground as garbage, which undoubtedly gives the already scarce cultivated land.
Plastics not only bring convenience to people's lives, but also bring irreparable trouble to the environment. People call the disaster that plastics bring to the environment "white pollution".
At present, many countries use incineration (heat energy recovery) or reprocessing (product recovery) to treat waste plastics. These two methods can recycle waste plastics and save resources. However, because waste plastics will produce gases harmful to human body and pollute the environment when they are burned or reprocessed, it can be said that the treatment of waste plastics is still a headache in environmental protection work.