Ethylene. Ethylene is a chemical compound described by the formula C2H4, a colorless gas with a faint odor. It is the simplest alkene (olefin). Contains a double bond and therefore belongs to unsaturated compounds and is highly reactive. Ethylene practically does not occur in nature. It is formed in small quantities in the tissues of plants and animals as an intermediate metabolic product. Plays an extremely important role in industry, the most produced organic compound in the world.
Currently, the main source of ethylene production is the pyrolysis of gaseous and liquid saturated hydrocarbons: ethane, propane and straight distilled gasoline.
Properties of ethylene:
Chemical formula H2C=CH2
Molecular weight 28.05
State - gaseous
Melting point 103.8 K (-169.2°C)
Boiling point 169.3 K (-103.7°C)
Density at normal conditions 1.26 kg/m 3
Density of liquid ethylene at 163.2 K (-109.8 ° C) - 610 kg/m 3
Flammability temperature 728 K (455°C)
Ethylene purity. For polymerization, ethylene must be thoroughly purified from impurities. Impurities in ethylene are divided into two main groups - inert and active. An inert impurity present in a noticeable amount, for example 5-10%, reduces the ethylene concentration by a significant amount, given the low compressibility of ethylene.
Active impurities in ethylene, such as vinyl-type compounds, usually copolymerize with ethylene, change the properties of the resulting polymer and affect the rate of polymerization.
Depending on the impurity content technical specifications It is planned to produce three grades of liquefied ethylene: A, B and C. Ethylene grades A and B are used for the production of polyethylene and ethylene oxide. Ethylene grade B - for the production of other organic products. Liquefied ethylene must comply with the requirements and standards.
Catalysts (initiators). Molecular oxygen and organic peroxides are mainly used as catalysts for the polymerization of ethylene. Of the peroxides in industry, the most widely used are di-tert-butyl peroxide, tert-butyl perbenzoate, etc. The effect of the initiator depends on the degree and rate of its decomposition at a given temperature and on the ability of the resulting radicals to react with the monomer.
Another factor characterizing the initiator is the content of active oxygen, i.e. theoretical percentage of active oxygen in pure peroxide.
In dry form, peroxides are explosive; their solutions in organic solvents are more stable and less explosive. Storage of initiators must be carried out under certain temperature conditions.
The main properties of the most common peroxide initiators are described below.
Di-tert-butyl peroxide (C8H18O2)
Application temperature 513-553 K (240-280°C)
Molecular weight 146.2
Liquid, density 793 kg/m 3
Boiling point at 0.1 MPa - 463 K (190°C)
Peroxide is insoluble in water, soluble in most organic solvents
Storage temperature 298 K (20°C).
Tert-butyl perbenzoate (C11H14O3)
Application temperature 453-513 K (180-240°C)
Molecular weight 194
Liquid, density at 293 K (20°C) - 1040 kg/m 3
Boiling point at 0.1 MPa - 397 K (124°C)
Storage temperature 293 K (20°C).
HDPE has a general purpose and is characterized by a linear structure with minor branches from the main chain.
The absence of volumetric limitations makes it possible to produce a material with increased crystallinity, which can reach 80%.
Thanks to this, high performance properties of this polymer are achieved.
Compositional features of polyethylene low pressure is a qualitative improvement in the modification of base polyethylene HDPE 276-73.For the formation of such polyethylene, certain conditions are required:
- temperature regime at 120–150 °C;
- pressure readings below 0.1–2 MPa;
- presence of Ziegler-Natta catalysts. Example: mixture of TiCl4 and AlR3.
The polymerization process occurs in suspension under the conditions of the ion-coordination mechanism. As a result, polyethylene is formed with an average molecular weight of 80–300 thousand.
Basic physical and chemical properties
Low-density polyethylene corresponds to the formula (-CH2-CH2-)n. It is chemically resistant to aggressive chemical elements and has excellent dielectric properties.
The granulated form of low-density polyethylene is produced by polymerization. The density indicator with this technological process is more than 0.945 g/cm³. The granules are more crystalline and have a low degree of transparency. The melting point depends on the length of the polymer chains.
The high melting point in the manufacture of HDPE products is very energy-consuming. However, the performance characteristics of such products are excellent. They can withstand fairly harsh conditions and relatively high temperatures without causing mechanical damage.
Subjective disadvantages of products made from HDPE are dullness of the surface, some roughness and insufficient ductility. In addition, the film made of low-density polyethylene easily wrinkles and rustles.
The tendency to cold flow changes the size of the film over time under constant load.Industrial Applications
The features of HDPE, characterized by high strength, low elongation at break and increased frost resistance, make the scope of its application quite wide. In the household segment, HDPE is used in the production of a variety of kitchen utensils and household items.
In construction, this material has found wide application in the manufacture of water pipes and various building materials. Most commonly used in the packaging industry in the production of packaging containers and bottles.
Film extrusion makes it possible to produce bags for packaging, T-shirt bags and bags with a die-cut handle. Used in the production of a barrier layer for multilayer packaging materials, bubble wrap and garbage bags.
Produced in this way are used in gas supply systems, cold water supply and for the purpose of protecting electrical networks. They are used in drainage systems, external and internal, as well as in the form of casing pipes in wells. In addition, the extrusion process produces waterproofing sheets, product parts for the mechanical engineering industry, membranes for waterproofing work, conveyor belts and geocells.
The blowing method is used to produce a variety of films and containers. Using injection molding, consumer goods, two-piece and one-piece lids, container boxes, furniture fittings and almost 400 types of auto parts.
The result of rotomolding is the release of:
- tanks,
- barrel,
- mobile toilets,
- children's play complexes,
- road barriers,
- wells,
- septic tanks,
- waste collection points and overpasses.
HDPE producing countries
Consumption of polymer raw materials in Europe shows annual growth of 6%. The volume of the low-density polyethylene market in Russia is approximately 340 thousand tons/year, and the average annual growth is 30%.
Lukoil-neftekhim experts estimate production in the Russian Federation at 450 thousand tons of HDPE, of which 315 thousand tons/year is for domestic consumption. From 30 to 35% of the total volume of HDPE produced in Russia is exported.Almost 87% of the total volume of HDPE produced in Russia comes from the following enterprises: Stavrolen from Lukoil-neftekhimiya, 
"Tomskneftekhim" from AK "Sibur", "Kazanorgsintez", "Nizhnekamskneftekhim" and "Gazpromneftekhim Salavat". Last year, Russian enterprises reduced HDPE production by 18%. The main reason was the downtime of the Stavrolen enterprise.
The leading position in the global market is occupied by Univation Technologies. It is a joint brainchild of Exxon Mobil and Dow/Union Carbide, which are recognized world leaders in the production of polyolefins.
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Recycling
Repeated processing of HDPE changes the viscosity properties at the level of 5–10%, and the strength characteristics decrease by 10–20%. The use of polyethylene recycling does not significantly affect the strength and viscosity properties of HDPE. Viscosity properties can be easily adjusted by changing the temperature during casting.
On this moment large amounts of money are invested in improving the quality characteristics of HDPE. It is in this polyolefin that many modern manufacturers see the future.
The main industrial method for the production of LDPE is the free radical polymerization of ethylene in bulk at temperatures of 200-320 °C and pressures of 150-350 MPa. Polymerization is carried out in continuous installations of various capacities from 0.5 to 20 t/h.
The technological process for the production of LDPE includes the following main stages: compression of ethylene to reaction pressure; indicator dosing; modifier dosing; ethylene polymerization; separation of polyethylene and unreacted ethylene; cooling and purification of unreacted ethylene (return gas); granulation of molten polyethylene; Confectioning, including dehydration and drying of polyethylene granules, distribution into analysis bins and determination of the quality of polyethylene, batch formation in commodity bins, mixing, storage; loading polyethylene into tanks and containers; packaging in bags; additional processing - obtaining polyethylene compositions with stabilizers, dyes, fillers and other additives.
2.1. TECHNOLOGICAL DIAGRAMS.
LDPE production consists of synthesis units and pre-processing and additional processing units.
Ethylene from a gas separation unit or storage facility is supplied at a pressure of 1-2 MPa and at a temperature of 10-40 ° C to the receiver, where return low-pressure ethylene and oxygen are introduced into it (when used as an initiator). The mixture is compressed by an intermediate pressure compressor to 25-30 MPa. is connected to the return ethylene flow of intermediate pressure, compressed by a reaction pressure compressor to 150-350 MPa and sent to the reactor. Peroxide initiators, if used in the polymerization process, are introduced into the reaction mixture using a pump immediately before the reactor. In the reactor, ethylene polymerization occurs at a temperature of 200-320 C. This diagram shows a tubular-type reactor, but autoclave reactors can also be used.
The molten polyethylene formed in the reactor, together with unreacted ethylene (the conversion of ethylene into polymer is 10-30%), is continuously removed from the reactor through a throttling valve and enters the intermediate pressure separator, where a pressure of 25-30 MPa and a temperature of 220-270 ° C are maintained. Under these conditions, separation of polyethylene and unreacted ethylene occurs. Melted polyethylene from the bottom of the separator, together with dissolved ethylene, enters the low-pressure separator through a throttling valve. Ethylene (intermediate pressure return gas) from the separator passes through a cooling and purification system (refrigerators, cyclones), where stepwise cooling to 30 - 40 ° C occurs and low molecular weight polyethylene is released, and then supplied to the suction of the reaction pressure compressor. In the low-pressure separator at a pressure of 0.1-0.5 MPa and a temperature of 200-250 °C, dissolved and mechanically entrained ethylene (low-pressure return gas) is released from polyethylene, which enters the receiver through a cooling and cleaning system (refrigerator, cyclone) . From the receiver, low-pressure return gas compressed by a booster compressor (with a modifier added to it, if necessary) is sent for mixing with fresh ethylene.
Molten polyethylene from the low-pressure separator enters the extruder, and from it in the form of granules it is sent by pneumatic or hydraulic transport for packaging and additional processing.
It is possible to obtain some compositions in a primary granulation extruder. In this case, the extruder is equipped with additional units for introducing liquid or solid additives.
A number of additional units compared to the technological scheme for the synthesis of traditional LDPE have a technological scheme for the production of linear high-density polyethylene, which is a copolymer of ethylene with a higher a-olefin (1-butene, 1-hexene, 1-octene) and obtained by copolymerization using an anion-coordination mechanism under the influence of complex organometallic catalysts. Thus, ethylene entering the plant undergoes additional purification. After cooling and purification, a comonomer - a-olefin - is introduced into the return gas of intermediate pressure. After the reactor, a deactivator is added to prevent polymerization from occurring in the polymer-monomer separation system. The catalysts are fed directly into the reactor.
IN last years a number of foreign LDPE manufacturing companies organized the production of LLDPE at industrial LDPE installations, equipping them with the necessary additional equipment.
Granulated polyethylene from the synthesis unit, mixed with water, will be fed to a polyethylene dehydration and drying unit, consisting of a water separator and a centrifuge. The dried polyethylene enters the receiving hopper, and from it through an automatic scale into one of the analysis hoppers. Analysis bins are designed to store polyethylene for the duration of the analysis and are filled one by one. After determining the properties, polyethylene is sent using pneumatic transport to an air mixer, to a substandard product bunker or to commercial product bunkers.
In an air mixer, polyethylene is averaged in order to equalize its properties in a batch composed of products from several analysis bins.
From the mixer, polyethylene is sent to the bunkers of the commercial product, from where it is supplied for shipment to railway tanks, tank trucks or containers, as well as for packaging in bags. All bins are purged with air to prevent ethylene accumulation.
To obtain compositions, polyethylene from the commercial product bins enters the supply bin. Stabilizers, dyes or other additives are supplied to the supply hopper, usually in the form of a granular concentrate in polyethylene. Through dispensers, polyethylene and additives enter the mixer. From the mixer the mixture is sent to the extruder. After granulation in an underwater granulator, separation of water in a water separator and drying in a centrifuge, the polyethylene composition enters the commercial product bins. From the bins the product is sent for shipment or packaging.
A key feature of the molecular structure of high-density polyethylene, as Alita specialists note, is the branching of polymer bonds, which leads to the formation of an amorphous crystalline structure and a decrease in density.
Properties of high density polyethylene (HDPE):
- molecular weight: (50-1000)*10^3
- degree of crystallinity: 70-90%
- melt flow rate (g/10 min at 230 degrees): 0.1-15
- glass transition temperature: -120 degrees
- melting point: 130-140 degrees
- density: 0.94-0.96 g/cm3
- shrinkage (in the production of finished products): 1.5-2.0%.
Chemical properties
Both types of polyethylene are characterized by low vapor and gas permeability and high chemical resistance, depending on the density and molecular weight of the polymer.
Polyethylene does not enter into chemical reactions with alkalis, including concentrated ones, and with salt solutions. It is resistant to carboxylic acids, concentrated hydrochloric acid, hydrofluoric acid and a number of other acids, to alkalis and solvents, alcohols and gasoline, oils and vegetable juices.
Exposure to 50% nitric acid, chlorine and fluorine leads to the destruction of polyethylene. The heavier halogen bromine diffuses through polyethylene, as does iodine. Polyethylene does not dissolve in organic solvents, but can swell.
Physical properties
Polyethylene is elastic and impact-resistant, does not break when bent. It is a dielectric and has low absorption capacity. Odorless, physiologically neutral.
High-density polyethylene is a soft material, low-density polyethylene is more rigid, even hard.
Performance
Polyethylene retains its polymer structure when heated in a vacuum or an inert gas, but in air, polymer destructuring begins at a temperature of 80 degrees.
Polyethylene is characterized by the effect of photoaging under the influence of ultraviolet radiation (in particular, under the influence of direct sun rays). Therefore, in the manufacture of polyethylene products that can be exposed to prolonged exposure to sunlight, photostabilizers are used - from ordinary carbon black to highly effective benzophenone derivatives.
In its normal state, polyethylene is environmentally friendly, since it does not release any dangerous or harmful substances into the environment.
The main types of polyethylene and ethylene copolymers currently produced by the global petrochemical industry are:
Polyethylene
- High density polyethylene (low density polyethylene) - HDPE.
- Low density polyethylene (high density polyethylene) - LDPE.
- Linear low density polyethylene - LLDPE.
- Metallocene linear low density polyethylene - mLLDPE, MPE.
- Medium density polyethylene - MDPE.
- High molecular weight polyethylene - HMWPE VHMWPE.
- Ultra high molecular weight polyethylene - UHMWPE.
- Foaming polyethylene - EPE.
- Chlorinated polyethylene - PEC.
Ethylene copolymers
- Copolymer of ethylene and acrylic acid - EAA.
- Copolymer of ethylene and butyl acrylate - EBA, E/BA, EBAC.
- Copolymer of ethylene and ethyl acrylate - EEA.
- Copolymer of ethylene and methyl acrylate - EMA.
- Ethylene-methacrylic acid copolymer, Ethylene-methyl methyl acrylate copolymer - EMAA.
- Copolymer of ethylene and methyl methacrylic acid - EMMA.
- Copolymer of ethylene and vinyl acetate - EVA, E/VA, E/VAC, EVAC.
- Copolymer of ethylene and vinyl alcohol - EVOH, EVAL, E/VAL.
- Polyolefin plastomers - POP, POE.
- Ethylene ternary copolymers - Ethylene terpolymer.
Areas of use of polyethylene
Despite the fact that progress does not stand still and new polymer materials with outstanding properties appear every year, polyethylene still remains the most widely used polymer in the world.
To produce final products from polyethylene granules, any available plastic processing methods can be used. And most of these methods do not require highly specialized equipment. This compares favorably with polyethylene, for example, from polyvinyl chloride (PVC).
The extrusion method makes it possible to produce polyethylene films of the most for various purposes, sheet polyethylene, pipes and cables. Containers and vessels are manufactured using the extrusion blow molding method (in particular, plastic bottles). For the production of volumetric and hollow products, including packaging materials, various containers, household materials, toys, injection molding, rotational method, and thermo-vacuum molding are used.
Cross-linked polyethylene, chlorosulfonated and foamed polyethylene are widely used in construction. Polyethylene with metal reinforcement, as Alita specialists note, can be used as a structural building material.
Polyethylene can be welded by any means - resistance welding, friction, filler rod, hot gas. This significantly expands the possibilities of its use in the most different industries industry and construction. Dielectric properties polyethylene are especially valuable for the cable industry, as well as in the manufacture of electrical appliances and electronic devices.
But, without a doubt, the most important area of application of polyethylene is packaging. Different types This material is suitable for both industrial, wholesale and retail packaging of goods and cargo. Polyethylene is used for packaging and packaging of industrial and food products. On the one hand, it is cheap, and on the other, it perfectly protects packaged products from any external influences in transit and during storage, and retail trade- allows you to effectively show off the product thanks to transparency and the availability of decorative effects.
There are many pigments designed for coloring polyethylene and packaging, as well as other products made from colored polyethylene, are widely popular.
Nowadays, as Alita specialists note, new areas of use are opening up for polyethylene. The creation of ultra-high molecular weight polyethylene opened the way for polymers to those areas where previously only metals or ceramics could be used.
Polyethylene with a supermolecular structure has unique properties. It is extremely durable and can be used at temperatures from -260 to +120 degrees. At the same time, it has an extremely low coefficient of friction and extremely high wear resistance. Therefore, ultra-high molecular weight polyethylene is an ideal material for the manufacture of parts of rotating devices - shafts, rollers, gears, bushings. It is also used in construction.
New varieties of polyethylene have made a real revolution in medicine. They are used to make durable prosthetic joints and bones that are not rejected by the body and allow long time maintain mobility and normal quality of life for people with severe injuries and diseases of the musculoskeletal system.
A valuable advantage of polyethylene (including in comparison with PVC and many other polymers) is the ease of its recycling, that is, recycling. With an established recycling collection system, pollution can be significantly reduced environment remnants of used polyethylene. Almost all polyethylene can be returned to production. At the same time, the consumption of primary petrochemical raw materials, which, as is known, has been constantly rising in price in recent years, is being reduced.
Since polyethylene entered the everyday life of people around the world, it has become one of the symbols of a comfortable life. And it is unlikely that any other materials will take over the palm among polymers in the near future. This amazing material combines too many advantages and benefits.
| LDPE Polyethylene/General purpose thermoplastics | HDPE Polyethylene/Polyolefins/General purpose thermoplastics | |
| Structure | Crystallizing material. | Crystallizing material. |
| Operating temperature | Material with short-term heat resistance of some grades up to 110 °C. Allows cooling down to -80 °C. Melting point of grades: 120 - 135 °C. | Material with short-term heat resistance without load up to 60 °C (for some brands up to 90 °C). Allows refrigeration (various brands ranging from -45 to -120 °C). |
| Mechanical properties | Characterized by good impact strength compared to HDPE. High creep is observed under prolonged loading. | Prone to cracking under loading. |
| Electrical properties | Has excellent dielectric characteristics. | Has excellent dielectric characteristics. Weather resistance. Not resistant to UV radiation. |
| Chemical resistance | It has very high chemical resistance (more than HDPE). | Has very high chemical resistance. Not resistant to fats and oils. |
| Food Contact | Allowed. Biologically inert. | |
| Recycling | Easily recyclable. | Easily recyclable. Does not differ in dimensional stability. |
| Application | One of the most widely used general purpose materials. | |
| Notes | Properties are highly dependent on the density of the material. An increase in density leads to an increase in strength, rigidity, hardness, and chemical resistance. At the same time, as density increases, impact resistance decreases. low temperatures, elongation at break, permeability to gases and vapors. Gives a shiny surface. Closest analogues: polyethylene, polyolefins. | Properties are highly dependent on the density of the material. An increase in density leads to an increase in strength, rigidity, hardness, and chemical resistance. At the same time, as density increases, impact resistance at low temperatures, elongation at break, resistance to cracking, and permeability to gases and vapors decrease. It is distinguished by increased radiation resistance. Closest analogues: polyethylene, polyolefins. |
Russian-made polyethylene
In Russia and the CIS countries, both Russian and international designations are used for the main types of polyethylene. Thus, the letters LDPE, PELD and PEBD denote high-density polyethylene (LDPE, LDPE), and HDPE or PEHD - respectively, low-density polyethylene (HDPE).
But in addition to these most common types of polyethylene, the modern chemical industry also produces other polymers of the same series, including those that appeared quite recently in the wake of the development of new technologies.
Thus, medium-density polyethylene (MDPE) has the international designation PEMD, and linear low-density polyethylene (LLDPE) - LLDPE or PELLD.
Many new materials do not have standard domestic designations, and Russian market they are present under English abbreviations. These are, in particular:
- LMDPE - linear medium density polyethylene
- VLDPE - very low density polyethylene
- ULDPE - ultra-low density polyethylene
- HMWPE or PEHMW - high molecular weight polyethylene
- HMWNDRE - high molecular weight high density polyethylene
- PEUHMW - supermolecular
- UHMWHDRE - ultra-high molecular structure polyethylene
Other frequently encountered designations include the following:
- REX, XLPE- cross-linked polyethylene
- EPE- foaming
- PEC, CPE- chlorinated
- MPE– low-density polyethylene made using metallocene catalysts.
Russian state standards provide for a digital classification of polyethylene grades produced by the domestic industry. The eight-digit designation contains information about the type of material, the method of its manufacture, the serial number of the brand, the density group and the flow rate. As Alita specialists note, these eight numbers can be supplemented with an indication of GOST, in accordance with which the material was produced.
Thus, brand 21008-075 indicates that this is a suspension-type HDPE made using organometallic catalysts, having a density of 0.948-0.959 g/cm3 and a fluidity of 7.5 g/10 min.
And brand 11503-070 is high-density polyethylene, without homogenization (this is indicated by the fourth digit - 0), with a density of 0.917-0.921 g/cm3 and fluidity - 7 g/10 min.
A marking of five digits is also used, where the first three are the polyethylene brand number, and the two digits after the dash are the additive formulation.
The designation of the polyethylene brand may also indicate the grade, color of the dyed material and additional information (for example, additional numbers indicating that this polyethylene is intended for use in the food industry or is suitable for the production of children's toys).
If the polyethylene composition is intended for the production of cables, this may be indicated by the letter “K” after the base brand number - for example, 10209K GOST 16336-77.
However, today many Russian manufacturers use their own or international product labeling.