What is polyethylene? What are its characteristics? How is polyethylene produced? These are very interesting questions that will be definitely considered in this article.
general information
Polyethylene is a chemical substance that is a chain of carbon atoms, each of which is attached to two hydrogen molecules. Despite the presence of the same composition, there are still two modifications. They differ in their structure and, accordingly, in their properties. The first is a linear chain in which the degree of polymerization exceeds five thousand. The second structure is a branching of 4-6 carbon atoms, which are attached to the main chain in an arbitrary way. How, in general terms, is linear polyethylene produced? This is achieved through the use of special catalysts, which affect polyolefins at a moderate temperature (up to 150 degrees Celsius) and pressure (up to 20 atmospheres). But what is he like? We know its chemical properties, and what are its physical properties?
What is he like?
Polyethylene is a thermoplastic polymer in which the crystallization process is carried out at a temperature of less than 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 is first carried out, 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, then nitric and sulfuric acids can destroy it quite quickly. 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?
To do this, use:
- High pressure method (low density). 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.
- The method of low pressure (high density). 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 separately there is a linear polyethylene production cycle , which was mentioned above. It is intermediate between the second and first points.
It should be noted that these are not the only technologies that are used. So, the use of metallocene catalysts is also quite common. The meaning of this technology is that through it they achieve a significant mass of polymer, while increasing the strength of the product. Depending on what structure and properties are needed when using one monomer, the method of preparation is selected. It may also be affected by requirements for melting point, 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 larger it is, the less crystallinity and higher elasticity of the polymer. Why is it important? The fact is that the mechanical properties of polyethylene grow together with its density and molecular weight. Let's look at a small example. Polyethylene sheet has significant rigidity and non-transparency. But if the low density method is used, then the resulting material will have relatively good flexibility and relative visibility through it. Why is such a diverse range available? Due to differences in operating conditions. So, polyethylene copes well with shock loads. He also tolerates frosts well. The operating temperature range of this material is from -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.
Material specificity
One significant drawback should be noted - the rapid aging of polyethylene. But this is fixable. The increase in service life is achieved thanks to special antioxidant additives, which may be carbon black, phenols or amines. It should also be noted that low-density material is more viscous, making it easier to process into products. It is impossible 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 tangent of the loss angle change only slightly 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, this helps to increase the dielectric loss tangent, which leads to some deterioration in the insulating properties. Well, now we have considered the general situation. Now let's pay attention to the specifics.
What is low pressure polyethylene?
This is an elastic light crystallizing material, the heat resistance of which is in the range from -80 to +100 degrees Celsius. It has a shiny surface. Glass transition starts at -20. And melting is in the range of 120-135. Characteristic is good impact strength and heat resistance. The density of polyethylene significantly affects the properties obtained. So, along with it, strength, stiffness, hardness and chemical resistance grow. But at the same time, the tendency to stretch and permeability to vapors and gases decreases. It should be noted creep, which is observed with prolonged load. 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 packages and containers. So, about a third of the production goes to create blow molding containers, which are used in the food industry, cosmetics, automotive, household, energy fields and films. But you can meet it when creating pipes and piping parts. An important advantage of such a material is its durability, low cost and ease of welding.
High pressure polyethylene
This 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 obtained material. This is an increase in strength, hardness, stiffness and chemical resistance. At the same time, the thickness of polyethylene adversely affects impact resistance, elongation, resistance to cracks and permeability to vapors and gases. In addition, it does not differ in dimensional stability and a noticeably negative effect at relatively small loads. It should be noted a really high chemical resistance and excellent dielectric characteristics. From the negative - fats, oils and ultraviolet radiation badly affect such polyethylene. Biologically inert, easy to recycle. It can also be characterized as resistant to radiation. The use of high-pressure polyethylene can be most often found 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. It can withstand temperatures up to 118 degrees Celsius. Another important advantage of this material is its resistance to cracking, heat resistance and impact strength. It is used for the manufacture of packaging, containers and containers. What does this polyethylene offer? The characteristics of this material are very high compared to the analog obtained by the low pressure method. Therefore, it has pretty good properties. But still, as a rule, it cannot 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 sheet and film polyethylene. These forms can be made of material of any density. Although there are still certain preferences. So, to obtain elastic and thin films, the low-pressure approach is widely used. The width of the material obtained, as a rule, reaches 1400 millimeters, and the length is 300 meters. Linear and high-pressure 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 but 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 about the requirements for the materials from which films, packaging and other various products are made. Moreover, it should be noted a wide range of applications of the resulting product and its species diversity. So, for example, reinforced polyethylene films are very common. Their feature is that at the same thickness they are a cut above in their properties than ordinary product samples. From the same reinforced plastic films make tablecloths, bags and many other useful things. And their properties are obtained through the introduction of special threads from natural or synthetic fibers.