Polyethylene is what? Application of polyethylene

What is polyethylene? What are his characteristics? How is polyethylene produced? These are very interesting questions that will be considered in this article.

general information

Polyethylene is a chemical substance, which is a chain of carbon atoms, to each of which two hydrogen molecules are attached. 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, which is attached to the main circuit in an arbitrary way. How does the linear polyethylene in general outline? This is achieved through the use of special catalysts that affect polyolefins at moderate temperatures (up to 150 degrees Celsius) and pressures (up to 20 atmospheres). But what is he like? We know its chemical properties, and what are physical?

What is he like?

Polyethylene is a thermoplastic polymer in which the crystallization process takes place at a temperature of less than minus 60 degrees Celsius. It is not transparent in a thick layer, it is not wetted with water, organic solvents do not influence it at room temperature. If the temperature exceeds plus 80 degrees Celsius, then first swell, and then decomposition into aromatic hydrocarbons and halogenated derivatives. Polyethylene is a substance that successfully resists the negative effects of solutions of acids, salts and alkalis. But if the temperature exceeds 60 degrees Celsius, it can quickly destroy nitric and sulfuric acids. For gluing polyethylene products, they can be treated with oxidants, followed by the application of the necessary substances.

How is polyethylene produced?

To do this, use:

• Method of high pressure (low density). Polyethylene is created at a high pressure, which is in the range of 1,000 to 3,000 atmospheres at a temperature of 180 degrees Celsius. The initiator is oxygen.
• Method of low pressure (high density). In this case, polyethylene is created at a pressure that is at least five atmospheres and a temperature of 80 degrees Celsius using an organic solvent and Ziegler-Natta catalysts.
• And separately there is a cycle of manufacture of linear polyethylene about which it was spoken above. It is intermediate between the second and the first points.

It should be noted that these are not the only technologies that are applied. So, the use of metallocene catalysts is quite widespread. The meaning of this technology lies in the fact that it achieves a significant amount of polymer by means of it, while increasing the strength of the product. Depending on what kind of structure and properties are needed when using one monomer, the selection method is selected. Also, this may be affected by the requirements for melting temperature, strength, hardness and density.

Why is there a big difference?

The main reason for the difference in properties is the branching of macromolecules. So, the more it is, the less the crystallinity and the higher the elasticity of the polymer. Why is it important? The fact is that the mechanical characteristics of polyethylene grow with its density and molecular mass. Let's look at a small example. Polyethylene sheet has a significant stiffness and not transparency. But if a low-density method is used, the resulting material will have relatively good flexibility and relative visibility through it. Why is such a variety available? Due to differences in operating conditions. Thus, polyethylene copes well with shock loads. He also tolerates frosts well. The operating temperature range of this material is -70 to +60 Celsius. Although individual brands are also 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.

Specificity of the material

One important drawback is the rapid aging of polyethylene. But this business is fixable. Increase in service life is achieved due to special additives, antioxidants, in the role of which can act gas black, phenols or amines. It should also be noted that the low density material is more ligated, so that it can be more easily processed into products. It is impossible not to mention the electrical properties. Polyethylene due to its non-polar polymer is a high-quality high-frequency dielectric. Due to this, the permeability and tangent of the loss angle vary slightly from changes in humidity, temperature (in the range from -80 to +100), and the frequency of the electric field. There should be noted one feature. Thus, if there are catalyst residues in the polyethylene, this contributes to an increase in the tangent of the dielectric loss angle, which leads to a certain deterioration in the insulation properties. Well, now we considered the general situation. And now let's pay attention to the specifics.

What is low pressure polyethylene?

It is an elastic light crystallizing material, the heat resistance of which is in the range of -80 to +100 degrees Celsius. Has a shiny surface. Glass-cutting starts at -20. A melting - in the range of 120-135. Characteristic is good impact strength and heat resistance. Density of polyethylene significantly affects the properties obtained. So, along with it, strength, stiffness, hardness and chemical resistance increase. But at the same time, the propensity to stretch and the permeability to vapors and gases decreases. It is impossible not to note the creep, which is observed with a prolonged load. Such polyethylene is biologically inert, and it can be easily processed. Which is very useful in modern conditions. Speaking about the use of polyethylene, it should be noted that it is used for the production of packages and containers. So, about a third of the production goes to create containers for the blow molding that are used in the food industry, cosmetics, automotive, household, energy fields and films. But you can meet it when creating pipes and pipe parts. An important advantage of such a material is its durability, cheapness and ease of welding.

Polyethylene of high pressure

It is an elastic light crystallizing material, the heat resistance of which (without load) is in the range from -120 to +90 degrees Celsius. Properties also strongly depend on the density of the material obtained. This leads to an increase in strength, hardness, rigidity and chemical resistance. Along with this, the thickness of polyethylene adversely affects the impact resistance, elongation, resistance to cracks and permeability for vapors and gases. In addition, it does not differ in dimensional stability and a noticeably negative effect at relatively low loads. It should be noted really high chemical resistance and excellent dielectric characteristics. From the negative - this polyethylene is badly affected by fats, oils and ultraviolet radiation. Biologically inert, can be easily processed. It can also be described as resistant to radiation. The use of high-density polyethylene is most often encountered when creating 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 used for the production of packages, containers and containers. What does this polyethylene offer? The characteristics of this material are very high in comparison with the analog obtained by the low pressure method. Therefore, it has rather good properties. But still, as a rule, it can not be equal to high-pressure polyethylene.

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 shapes can be made of material of any density. Although there are certain preferences. Thus, for the production of elastic and thin films, the low pressure approach is widely used. The width of the received material, as a rule, reaches 1400 millimeters, and the length is 300 meters. Linear and high-density polyethylene are more stringent, so they are used for structures that should not be affected: the same sheets, pipes, molded and injection molded products and so on.

Conclusion

And finally, we can not 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 the issues associated with high-pressure polyethylene. It is even older and dates back to 1977. These normative documents contain information about the requirements for materials, from which films, packaging and other various products are manufactured. And it should be noted a wide range of applications of products and its species diversity. For example, reinforced polyethylene films are very common. Their peculiarity is that with the same thickness, they are superior in their properties to their properties than conventional samples of products. Of the same reinforced polyethylene films make tablecloths, bags and many other useful things. And their properties are obtained through the introduction of special yarns made from natural or synthetic fibers.