Polyethylene: melting point, consumer properties and application

Today, mankind can not do without artificial materials. They have a number of unique qualities, are available and significantly cheaper production. One such material is polyethylene. The melting point, as well as other technical characteristics, deserve careful consideration. After all, this is one of the most popular materials today. More than half of all ethylene produced by the world's 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. To each of them 2 hydrogen molecules are attached. In the world there are two modifications of this substance. They are different in structure. This is reflected in the properties that polyethylene possesses (melting point and boiling point, consumer properties). Only their origin unites them. Both modifications are obtained 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 consist of carbon atoms (from 4 to 6).

To create a linear polyethylene, special catalysts are used. The polymerization process proceeds at a temperature of up to 150 ° C.

Characteristics

A thermoplastic polymer, which is characterized by opacity at a thick layer, is polyethylene for us. The melting point, the 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 polyethylene with water. At home, it is not exposed to various organic solvents. It also does not react at room temperature with aqueous salt, acidic and alkaline solutions.

When the temperature rises to 60 ° C, the material becomes vulnerable to sulfuric and nitric acids. Using oxidants to treat the surface of polyethylene, the destruction of the surface layer should be expected. The material begins to be wetted with water. This quality is necessary for gluing polyethylene.

Methods of polymerization

Depending on the method of polymerization of ethylene, polyethylene is of 3 kinds: low, high pressure and linear type of material. This determines what qualities polyethylene will possess. The melting point, the technical properties of each species are different. Therefore, they are used practically in any sphere of human activity.

Polyethylene, manufactured under high pressure, 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 participates as an initiator.

Polyethylene of low pressure is made with the help of Ziegler-Natta catalysts. In this process, an organic solvent also participates. The working pressure is not less than 5 atm., And the temperature exceeds 80 ° С.

Linear (medium) polyethylene is an intermediate between the varieties considered. This concerns 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

Branched structure of macromolecules determines the properties that polyethylene possesses. The melting point and density depend on the type of chain. The more ramifications it has, the more elastic material with less crystalline properties is obtained at the outlet.

This feature of the structure makes it more difficult to form a denser package of macromolecules, becoming an obstacle to a 100% crystallinity level. The material also has an atmospheric phase. It contains insufficiently ordered sections of molecules. The method of production determines the ratio of the 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 more crystallinity (molecular weight), the higher the mechanical performance. Therefore, in the form of a film, the material is transparent and elastic. But the sheets of polyethylene will be tough and opaque.

The effect of temperature

Under the influence of the environment, the qualities with which polyethylene is endowed change. The melting point of this substance also depends on the mode of production. In the general form, when heating polyethylene passes several stages. At first it becomes softer, more elastic. It easily deforms under the influence of mechanical influences.

The brittleness temperature at which the average polyethylene loses its strength characteristics is 70 ° C. With its further increase, the substance softens even more. It completely loses the inherent shape when heated at 120 ° C. In a liquid substance, it is converted 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 street products, it is necessary to choose stronger varieties. Otherwise, soft, elastic polyethylene after a year of operation under direct sunlight will become hard and brittle. Even the color of the material changes with time.

Low-pressure polyethylene

Each type of material has special qualities. This expands the range of applications that polyethylene possesses. The melting point (high density) is 120-135 ° C. For individual brands, the heat resistance is 110 ° C. High molecular density contributes to increased thermal and impact resistance.

In addition to these qualities, low pressure polyethylene is less susceptible to chemical attack. However, excessive density of molecules at low temperatures makes the material brittle, it becomes permeable to vapors, gases.

This kind of material has good dielectric characteristics. It is biologically inactive, but easily processed in industrial production.

Polyethylene of high pressure

This group includes elastic, light polyethylene. The melting point, the properties of crystallization do not allow performing high-strength, heat-resistant products from it. Depending on the brand can have a different density. Their melting point is from 60 to 90 ° C.

Just like the previous type of material, high-density polyethylene is more durable if the molecular weight increases. It becomes less susceptible to chemical, ultraviolet influences. But this reduces its ability to withstand shocks. On such polyethylene in severe frosts cracks appear, breaks. It becomes permeable to vapors and gases.

Such material also has good dielectric properties. He does not show resistance to fats, oil. But this material is able to contain radiation rays. Biologically, this material is also inert, but easy to process.

Application of polyethylene of low pressure

The quality inherent in the material determines the field of application that polyethylene has. The melting point (the use of this indicator is mandatory in the selection of each product) makes it possible to make packaging and packaging from such a substance. Most often, the container is manufactured by blow molding. These can be containers for cosmetics or perfumes, food containers.

Canisters and containers of low-density 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 displace other competitive products from the market.

Application of high-density polyethylene

Polyethylene, the melting point of which is lower than that of the previous version, is used in the production of films for agriculture, food industry and other technical purposes. Its 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, packets all over the world are consumed every year on a large scale. This kind of material has displaced from the main market segments products from other materials.

Structure of consumption

Polyethylene, the melting point of which determines the field of its application, is in great demand all over the world. The structure of consumption of the material is quite interesting. 60-70% of polyethylene is used for making sheets and films.

Also a rather large part of the total volume of production is occupied by products obtained by injection molding or by extrusion. More insignificant production of insulation for electrical wires, pipes and fittings. Also, polyethylene is used to produce products by blowing and the like.

In the production of sheets and films, high-density polyethylene (low density) is almost always used. They are manufactured in different ways. The thickness of the films is in the range 0.03-0.3 mm, and the sheets - 1-6 mm.

In addition to packaging, such material can produce bags, bags, liners for boxes, boxes and other containers. The properties that a product must possess are determined by the way the polyethylene is produced. At the end of production, each type of material is given a brand. It helps to choose the right kind of material for any industry.