Polyethylene - what is it? Application of polyethylene

2024 Author: Howard Calhoun | [email protected]. Last modified: 2023-12-17 10:16

What is polyethylene? What are its characteristics? How is polyethylene produced? These are very interesting questions that will definitely be addressed in this article.

General information

Polyethylene is a chemical substance that is a chain of carbon atoms, each of which has two hydrogen molecules attached to it. Despite the presence of the same composition, there are still two modifications. They differ in their structure and, accordingly, properties. The first is a linear chain in which the degree of polymerization exceeds the figure of five thousand. The second structure is a branch of 4-6 carbon atoms that are attached to the main chain in an arbitrary way. How is linear polyethylene obtained in general terms? This is achieved through the use of special catalysts that affect polyolefins at moderate temperature (up to 150 degrees Celsius) and pressure (up to 20 atmospheres). But what does he represent? We know its chemical properties, but what are its physical properties?

What is it?

Polyethylene is a thermoplastic polymer in which the crystallization processcarried out at temperatures below minus 60 degrees Celsius. It is not transparent in a thick layer, is not wetted by water, and organic solvents do not affect it at room temperature. If the temperature exceeds plus 80 degrees Celsius, then swelling first occurs, and then decomposition into aromatic hydrocarbons and halogen derivatives. Polyethylene is a substance that successfully resists the negative effects of solutions of acids, s alts and alkalis. But if the temperature exceeds 60 degrees Celsius, then nitric and sulfuric acids can quickly destroy it. For gluing products made of polyethylene, they can be treated with oxidizing agents, followed by the application of the necessary substances.

How is polyethylene produced?

Use for this:

• High pressure (low density) method. Polyethylene is created at high pressure, which is in the range from 1,000 to 3,000 atmospheres at a temperature of 180 degrees Celsius. Oxygen acts as an initiator.
• Low pressure (high density) method. In this case, polyethylene is created at a pressure of at least five atmospheres and a temperature of 80 degrees Celsius using an organic solvent and Ziegler-Natta catalysts.
• And there is a separate production cycle of linear polyethylene, which was mentioned above. It is intermediate between the second and first points.

Note that these are not the only technologies being used. So,The use of metallocene catalysts is also quite common. The meaning of this technology lies in the fact that through it a significant mass of polymer is achieved, while increasing the strength of the product. Depending on what structure and properties are needed when using one monomer, the choice of a method of obtaining occurs. It may also be affected by melting temperature, strength, hardness and density requirements.

Why is there such a big difference?

The main reason for the difference in properties is the branching of macromolecules. So, the larger it is, the less crystallinity and the higher the elasticity of the polymer. Why is it important? The fact is that the mechanical properties of polyethylene grow along with its density and molecular weight. Let's look at a small example. Polyethylene sheet has significant rigidity and opacity. But if a low density method is used, then the resulting material will have relatively good flexibility and relative visibility through it. Why is there such a wide variety of products? due to different operating conditions. So, polyethylene copes well with shock loads. It also tolerates frost well. The operating temperature range of this material is from -70 to +60 Celsius. Although individual brands are adapted for a slightly different gradient - from -120 to +100. This is affected by the density of polyethylene and its structure at the molecular level.

Material specifics

It should be noted one significant drawback - the rapid aging of polyethylene. But this is fixable. The increase in service life is achieved thanks to special antioxidant additives, which can be carbon black, phenols or amines. It should also be noted that the low density material is more viscous, so that it can be more easily processed into products. Not to mention the electrical properties. Polyethylene, due to the fact that it is a non-polar polymer, is a high-quality high-frequency dielectric. Due to this, the permeability and the loss tangent change little from changes in humidity, temperature (in the range from -80 to +100) and the frequency of the electric field. One feature should be noted here. So, if there are catalyst residues in polyethylene, then this contributes to an increase in the dielectric loss tangent, which leads to some deterioration in the insulating properties. Well, now we have considered the general situation. Now let's get specific.

What is LDPE?

This is an elastic, lightweight, crystallizing material with heat resistance ranging from -80 to +100 degrees Celsius. Has a shiny surface. The glass transition starts at -20. And melting is in the range of 120-135. It is characterized by good impact strength and heat resistance. The density of polyethylene significantly affects the properties obtained. So, along with it, strength, rigidity, hardness and chemical resistance increase. But at the same time, the tendency to stretch and permeability decreases.for vapors and gases. It is impossible not to note the creep that is observed during prolonged loading. Such polyethylene is biologically inert and can be easily recycled. Which is very useful in modern conditions. Speaking about the use of polyethylene, it should be noted that it is used for the manufacture of packaging and containers. So, about a third of the production goes to create blow molding containers that are used in the food industry, cosmetics, automotive, household, energy and films. But you can also meet it when creating pipes and pipeline parts. An important advantage of this material is its durability, low cost and ease of welding.

HDPE

This is an elastic, lightweight, crystallizing material with a heat resistance (without load) ranging from -120 to +90 degrees Celsius. The properties also strongly depend on the density of the resulting material. This results in an increase in strength, hardness, stiffness and chemical resistance. At the same time, the thickness of the polyethylene negatively affects the impact resistance, elongation, crack resistance and permeability to vapors and gases. In addition, it is not dimensionally stable and has a noticeable negative effect at relatively small loads. It should be noted the really high chemical resistance and excellent dielectric characteristics. From the negative - such polyethylene is badly affected by fats, oils and ultraviolet radiation. Biologically inert, can be easily recycled. It is also possibleto characterize and as resistant to radiation. The use of high-density polyethylene is most common in the creation of technical, food and agricultural films. Although, of course, this is not the only option.

Linear polyethylene

It is an elastic crystallizing material. Can withstand temperatures up to 118 degrees Celsius. Also an important advantage of this material is its resistance to cracking, heat resistance and impact strength. It is applied to production of packings, capacities and containers. What does this polyethylene offer? The characteristics of this material are very high in comparison with the analogue obtained by the low pressure method. Therefore, it has quite good properties. But still, as a rule, it cannot be equal to HDPE.

How can the material be presented?

So, we have already considered the main types of polyethylene. In what form is it created? The most popular are polyethylene sheet and film. These molds can be made from any material density. Although there are still certain preferences. Thus, the low pressure approach is widely used to obtain elastic and thin films. The width of the resulting material, as a rule, reaches 1400 millimeters, and the length is 300 meters. Linear and high-density polyethylene are more rigid, so they are used for structures that should not be affected: the same sheets, pipes, molded and molded products, etc.

Conclusion

And finally, one cannot fail to mention the regulatory documents, according to which polyethylene is produced. GOST 16338-85 is responsible for products that are created at low pressure. It has been operating since 1985. GOST 16337-77 regulates issues related to high pressure polyethylene. It is even older and dates back to 1977. These regulatory documents contain information on the requirements for the materials from which films, packaging and various other products are made. Moreover, it should be noted a wide range of application of the products obtained and its species diversity. So, for example, reinforced polyethylene films are very common. Their peculiarity is that, with the same thickness, they are a cut above in their properties than ordinary product samples. Tablecloths, bags and many other useful things are made from the same reinforced polyethylene films. And their properties are obtained through the introduction of special threads from natural or synthetic fibers.