In the history of science, some discoveries happened by chance, and the materials demanded today are often a by-product of some experience. Quite by chance, aniline dyes were discovered for fabrics, which subsequently gave an economic and technical breakthrough in light industry. A similar story happened with polyethylene.
Material discovery
The first case of polyethylene production occurred in 1898. During the warming up of diamezotan, a chemist of German origin Hans von Pehman discovered a strange sediment at the bottom of the tube. The material was quite dense and resembled wax, colleagues of the scientist called it polymethilline. This group of scientists didn’t go further than chance, the result was almost forgotten, no one had any interest. But still the idea hung in the air, requiring a pragmatic approach. And so it happened, after thirty odd years, polyethylene was rediscovered as a random product of an unsuccessful experiment.
The British pick up and win
Modern polyethylene material was born in the laboratory of the British company Imperial Chemical Industries. E. Fossett and R. Gibson conducted experiments involving high and low pressure gases and noticed that one of the nodes of the equipment in which the experiments were carried out was covered with an unknown waxy substance. Interested in a side effect, they made several attempts to obtain the substance, but to no avail.
Synthesis of the polymer failed M. Perrin, an employee of the same company, two years later. It was he who created the technology that served as the basis for the industrial production of polyethylene. In the future, the properties and qualities of the material were changed only by using various catalysts. Mass production of polyethylene began in 1938, and it was patented in 1936.
Raw materials
Polyethylene is a solid polymer of white color. Belongs to the class of organic compounds. What is polyethylene made of? The raw material for its production is ethylene gas. The gas is polymerized at high and low pressure, and granules of raw materials are obtained at the outlet for further use. For some technological processes, polyethylene is produced in powder form.
Main types
To date, the polymer is produced in two major grades of PVD and PNP. A material manufactured at an average pressure is a relatively new invention, but in the future the quantity of the produced product will invariably grow due to improved characteristics and a wide field for application.
For commercial use, the following types of material (classes) are produced:
- Low density or another name - high pressure (LDPE, LDPE).
- High density, or low pressure (LDPE, PNP).
- Linear polyethylene, or medium pressure polyethylene.
There are also other types of polyethylene, each of which has its own properties and scope. In the production process, various dyes are added to the granular polymer to produce black polyethylene, red or any other color.
LDPE
The production of polyethylene is the chemical industry. Ethylene gas is the main element (of which polyethylene is made), but not the only one required to obtain the material.
The production of high-pressure polyethylene occurs in autoclaves, tubular reactors. There are eight brands of LDPE manufactured in an autoclave according to GOST. Twenty-one types of high pressure polyethylene are produced from the tubular reactor.
For the synthesis of PVP, the following conditions are required:
- The temperature regime is from 200 to 250 ° C.
- The catalyst is pure oxygen, peroxide (organic).
- Pressure from 150 to 300 MPa.
The cured mass in the first phase has a liquid state, after which it is transferred to a separator, then to a granulator, where granules of the finished material are formed.
The qualities of LDPE are used for the production of packaging films, thermofilms, multilayer packaging. Also, high-pressure polyethylene is used in the automotive, chemical, food industries. It makes high-quality durable pipes used in the residential sector.
Linear polyethylene
What is medium pressure polyethylene or linear polyethylene made of?
- The heating temperature is up to 120 ° C.
- Pressure mode up to 4 MPa.
- The process stimulator is a catalyst (Ziegler-Natta, a mixture of titanium chloride with an organometallic compound).
The process is accompanied by the precipitation of polyethylene in the form of flakes, which then go through the process of separation from the solution with subsequent granulation.
This type of polyethylene is characterized by a higher density, resistance to heat and tear. Scope of application are various types of packaging films, including for packaging of hot materials / products. From granular raw materials of this type of polymer, parts are made for large-sized machines by casting, insulation materials, pipes of increased strength, consumer goods, etc.
Low-pressure polyethylene
The production of EOR has three methods. Most enterprises use the "suspension polymerization" method. The process of obtaining the PNP occurs with the participation of the suspension and constant mixing of the feedstock, a catalyst is required to start the process.
The second most common production method is polymerization in solution under the influence of temperature and the participation of a catalyst. The method is not very effective, because during the polymerization, the catalyst reacts, and the final polymer loses some of its qualities.
The last of the methods for the production of PNP is gas-phase polymerization, it is almost a thing of the past, but is sometimes found in individual enterprises. The process occurs by mixing the gas phases of the feed under the influence of diffusion. The final polymer is obtained with a heterogeneous structure and density, which affects the quality of the finished product.
The production of low-pressure polyethylene occurs in the following mode:
- The temperature is maintained between 120 ° C and 150 ° C.
- Pressure should not exceed 2 MPa.
- Catalysts for the polymerization process (Ziegler-Natta, a mixture of titanium chloride with an organometallic compound).
The material of this manufacturing method is characterized by rigidity, high density, low elasticity. Therefore, its scope is industry. Technical polyethylene is used for the manufacture of large containers with enhanced strength characteristics. It is in demand in the construction sector, the chemical industry, it is almost not used for the production of consumer goods.
The properties
Polyethylene is resistant to water, to many types of solvents, acids (organic, inorganic), does not react with salts. When burning, the smell of paraffin is released, there is a glow of a blue tint, the fire is weak. Decomposition occurs when exposed to nitric acid, chlorine and fluorine in a gaseous or liquid state. During aging, which occurs in air, cross-links between the chains of molecules form in the material, which makes the material brittle, crumbling.
Consumer qualities
Polyethylene is a unique material familiar in everyday life and production. It is unlikely that an ordinary consumer will be able to determine how many items from it he encounters daily. In the global production of polymers, polyethylene occupies the lion's share of the market - 31% of the total gross product.
Depending on what polyethylene and production technologies are made of, its qualities are determined. This material sometimes combines opposite indicators: flexibility and strength, ductility and hardness, strong tensile and tear resistance, resistance to aggressive environments and biological agents. In everyday life, we use bags of various densities, disposable tableware, plastic covers, parts for household appliances and much more.
Areas of use
The use of polyethylene products has no restrictions, any industry or human activity is accompanied by this material:
- The polymer is most widely used in the manufacture of packaging materials. This part of the application accounts for about 35% of all raw materials produced. Such use is justified by dirt-repellent properties, lack of environment for the occurrence of fungal infections and the activity of microorganisms. One of the most successful finds is a polyethylene sleeve, which is widely used. By varying the length at his own discretion, the user is limited only by the width of the package.
- Remembering what polyethylene is made of, it becomes clear why it became widespread as one of the best insulation materials. One of its sought-after qualities in this area was the lack of electrical conductivity. Its water-repellent properties are also indispensable, which has found application in the production of waterproofing materials.
- Resistance to the destructive power of water, as a solvent, allows the production of pipes from polyethylene for domestic and industrial consumers.
- In the construction industry, the soundproofing qualities of polyethylene and its low thermal conductivity are used. These properties were useful in the manufacture of materials based on it for the insulation of residential and industrial facilities. Technical polyethylene is used for insulation of thermal routes, in mechanical engineering, etc.
- The material is not less resistant to aggressive environments of the chemical industry, polyethylene pipes are used in laboratories and chemical industries.
- In medicine, polyethylene is useful in the form of dressings, limb prostheses, it is used in dentistry, etc.
Recycling methods
Depending on how the granular raw materials were processed, it will depend on which brand of polyethylene will be obtained. Common ways:
- Extrusion (extrusion). It is used for the manufacture of pipes, packaging and other types of films, sheet material for construction and decoration, the manufacture of cables, the sleeve is made of polyethylene and other products.
- Casting, thermo-vacuum molding . It is mainly used for the manufacture of packaging materials, boxes, etc.
- Extrusion Blow Molding, Rotary. Using this method, bulk containers, large containers, vessels are obtained.
- Reinforcement. According to a certain technology, reinforcing elements (metal) are laid in the formed mass of polyethylene, which makes it possible to obtain building materials of increased strength, but with a lower cost.
What is polyethylene made of besides the main constituent substances? A process catalyst and additives that change the properties and quality of the finished material are mandatory.
Recycling
Durability of polyethylene is its plus as a consumer product and its minus, as one of the main polluting factors. Today, recycling is becoming important. All grades of polyethylene can be disposed of and re-turned into granular raw materials, from which many popular consumer and industrial goods can be made.
Plastic lids, bags, bottles will decompose in a landfill for hundreds of years, and accumulated waste poisons natural vital resources. World practice shows an increase in the number of polyethylene processing enterprises. In fact, collecting garbage, in such companies carry out its reorganization, crush it. Thus, there is a saving of resources, environmental protection and the production of popular products.