Today, mankind cannot do without artificial materials. They have a number of unique qualities, are available and significantly reduce the cost of production. One such material is polyethylene. The melting point, as well as its other technical characteristics, deserve detailed consideration. After all, this is one of the most popular materials today. More than half of all ethylene produced by the global chemical industry is sent to produce polyethylene. To understand why it is so popular today, you should consider its characteristics.
What is a substance
The structure of the polyethylene molecule is quite simple. It looks like a chain that consists of carbon atoms. Each of them is joined by 2 hydrogen molecules. There are two modifications of this substance in the world. They are different in structure. This is reflected in the properties possessed by polyethylene (melting and boiling points, consumer properties). Only their origin unites them. Both modifications are made from ethylene.
The first type of polyethylene consists of linear monomers. Their degree of polymerization is 5000 or more. The second modification has branching monomers. They are composed of carbon atoms (4 to 6).
To create linear polyethylene, special catalysts are used. The polymerization process is carried out at temperatures up to 150 ° C.
Characteristics
Thermoplastic polymer, which is characterized by opacity in a thick layer, appears to us as polyethylene. Melting point, technical features of the material make it popular. It crystallizes in the range from -60 to -269 ° C.
Its main positive quality is the absence of wetting of polyethylene with water. At home, it is not affected by various organic solvents. Also, it does not react at room temperature with aqueous saline, acid and alkaline solutions.
When the temperature rises to 60 ° C, the material becomes vulnerable to sulfuric and nitric acids. Using oxidizing agents to treat the surface of polyethylene, one should expect destruction of the surface layer. The material begins to get wet with water. This quality is necessary for bonding polyethylene.
Polymerization Methods
Depending on the method of polymerization of ethylene, polyethylene is of 3 types: low, high pressure and linear type of material. This determines what qualities polyethylene will have. Melting point, technical properties of each variety are different. Therefore, they are used in almost any field of human activity.
High pressure polyethylene is softer. It is polymerized by the radical method. The pressure at this reaches 1-3 thousand atm. The temperature is 180 ° C. Oxygen in this case is involved as an initiator.
Low pressure polyethylene is made using Ziegler-Natta catalysts. An organic solvent also takes part in this process. Working pressure is at least 5 atm., And the temperature exceeds 80 ° C.
Linear (medium) polyethylene is an intermediate material between the considered varieties. This applies to its qualities and properties. It is made at a pressure of 30-40 atm. When using metallocene catalysts, it is possible to obtain a product of enhanced strength.
The reason for the differences in the properties of polyethylene
The branching structure of macromolecules determines the properties that polyethylene possesses. Melting point, density depend on the type of chain. The more branches it has, the more elastic material with less crystalline properties is obtained at the output.
This structural feature complicates the formation of denser packing of macromolecules, becoming an obstacle to 100% crystallinity. The material also has an atmospheric phase. It contains insufficiently ordered sections of molecules. The production method determines the ratio of crystalline and atmospheric phases. It is this feature that affects the properties of polyethylene.
Therefore, films that are produced under low pressure are more permeable than their other varieties. The greater the crystallinity (molecular weight), the higher the mechanical properties. Therefore, in the form of a film, the material is transparent and elastic. But polyethylene sheets will be tough and opaque.
Temperature exposure
Under the influence of the environment, the qualities endowed with polyethylene change. The melting point of this substance also depends on the production method. In general, when heated, polyethylene goes through several stages. At first it becomes softer, more elastic. It is easy to deform under the influence of mechanical influences.
The fragility temperature at which medium polyethylene loses its strength characteristics is 70 ° C. With its further increase, the substance softens even more. It completely loses its previously inherent shape when heated to 120 ° C. It turns into a liquid substance at a temperature of 130 ° C.
In addition to the heating temperature, it is necessary to take into account the effect of ultraviolet radiation. If the material is used for outdoor products, it is necessary to choose more durable varieties. Otherwise, soft, flexible polyethylene after a year of operation in direct sunlight will become hard and brittle. Even the color of the material changes over time.
Low-pressure polyethylene
Each type of material has special qualities. This extends the range of applications that polyethylene has. The melting point (high density) is 120-135 ° C. For some brands, the heat resistance is 110 ° C. High molecular density enhances thermal and impact resistance.
In addition to the listed qualities, low-pressure polyethylene is less susceptible to chemical influences. However, the excessive density of molecules at low temperatures makes the material brittle, it becomes permeable to vapors and gases.
This kind of material has good dielectric characteristics. It is biologically inactive, but easily processed in industrial production.
High pressure polyethylene
This group includes elastic, lightweight polyethylene. The melting temperature, crystallization properties do not allow to perform high-strength, heat-resistant products from it. Depending on the brand, it may have a different density. Their melting point is from 60 to 90 ° C.
Just like the previous type of material, high-pressure polyethylene is more durable if the molecular weight increases. It becomes less susceptible to chemical, ultraviolet influences. But at the same time, its ability to withstand shock is reduced. On such polyethylene, in severe frosts, cracks, tears appear. It becomes permeable to vapors and gases.
Such material also has good dielectric properties. It does not show resistance to fats, oil. But this material is able to restrain radiation rays. Biologically, this material is also inert, but easy to recycle.
The use of low pressure polyethylene
The inherent qualities of the material determine the scope that polyethylene has. The melting temperature (the use of this indicator is mandatory when choosing each product) allows you to make packaging and containers from such a substance. Most often, containers are made by blow molding. It can be containers for cosmetics or perfumes, food packaging.
Canisters and containers made of low-pressure polyethylene are used in the automotive and chemical industries, in the manufacture of barrels and fuel tanks.
The production of packaging films from a similar material is gaining momentum. It is widely used in the manufacture of pipes, fittings. It is a cheap and durable material. It is able to oust other competitive products from the market.
The use of high pressure polyethylene
Polyethylene, the melting point of which is lower than that of the previous variety, is used in the manufacture of films for agriculture, food industry and other technical purposes. His demand is constantly growing.
Different films for agricultural purposes can have additional reinforcement, their color is also different. They are used in greenhouses, in the fields to improve the quality and volume of the crop.
Food films, bags all over the world are consumed every year on a large scale. This type of material displaced products from other materials from the main market segments.
Consumption pattern
Polyethylene, the melting point of which determines the scope of its application, is in great demand all over the world. The structure of material consumption is quite interesting. 60-70% of polyethylene is used for the manufacture of sheets and films.
Also, a fairly large part of the total production is occupied by products obtained by injection molding or by extrusion. More insignificant is the production of insulation for electric wires, pipes and fittings. Also, polyethylene is used to obtain products by blowing and other things.
In the manufacture of sheets and films almost always apply high-pressure polyethylene (low density). They are made in many ways. The film thickness is in the range of 0.03-0.3 mm, and the sheets are 1-6 mm.
In addition to packaging, bags, bags, lining for boxes, boxes and other containers can be made from such material. The properties that the product should possess determine the method of production of polyethylene. At the end of production, each type of material is assigned vintage. It helps to choose the right kind of material for any industry.