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Production of low pressure polyethylene. Delivery basis map

The first experience of ethylene polymerization at the end of the 19th century was received by a native of Russia - scientist Gustavson by carrying out this process with AlBr3 catalyst. For for long years polyethylene was produced in small volumes, but in 1938 the British mastered the industrial production process. At that time, the polymerization method was not yet perfect.

1952 made a breakthrough in the process of industrial production. The German chemist Ziegler invented an effective version of ethylene polymerization under the action of metal-organic catalysts. However, the real technology for the production of polyethylene is based on this method.

Raw material

The starting material for production is ethene, the simplest representative of a number of alkenes. The simplicity of this production method is highly dependent on the presence of ethyl alcohol, which is used as a raw material. Modern industrial lines for polymer production are developed taking into account their work on oil and associated gases– readily available fractions of oil.

Such gases are released during pyrolysis or cracking of petroleum products at very high temperatures and contain impurities of H2, CH4, C2H6 and other gases. Associated gas, in turn, contains components such as paraffin gases, so when exposed to them heat treatment ethylene is obtained in high yield.

High pressure polyethylene production technology

The process of obtaining PE proceeds according to a radical mechanism. When carrying out, various kinds of initiators are used to lower the activation threshold of the molecule. Examples of such are hydrogen peroxide, organic peroxides, O2, nitriles. The radical mechanism, in general, does not differ from conventional polymerization:

Stage 1 - initiation;
stage 2 - chain extension;
Stage 3 - open circuit.

The chain is initiated by the release of free radicals upon thermal treatment of their source. Ethene reacts with the released radical, is endowed with a certain Eact, thereby increasing the number of monomer molecules around itself. Subsequently, the chain grows.

Process technology

There are two options for the polymerization process - either polyethylene is formed in bulk or in suspension. The first received and is a collection of processes.

Ethylene gas, which is a mixture and not a pure substance, first passes through a filtration path through a fabric filter, which retains mechanical impurities. Further, the initiator is brought to the purified ethene in a cylinder, the volume of which is calculated based on the process conditions. The correction is made for the highest polymer yield.

After, the mixture is transported, filtered and subjected to compression in two stages. At the outlet of the reactor, almost pure polyethylene is obtained with an admixture of ethylene, which is eliminated by throttling the mixture in the receiver under low pressure.

Low pressure polyethylene production technology

The sources of raw materials for the production of this type of polyethylene are pure ethylene without impurities and a catalyst - aluminum triethylate and Ti tetrachloride. Al(C2H5)3 can be replaced by either diethylaluminum chloride or aluminum ethoxide dichloride. The catalyst is obtained in 2 stages.

Process technology

For this process of obtaining PE low pressure characterized by both periodicity and continuity. The process scheme also depends on the choice of technology, each of which is different in the design of equipment, the volume of reactors, the method of cleaning polyethylene from impurities, etc.

The most common polymer production scheme includes three continuous stages: raw material polymerization, product purification from catalyst residues and its drying. Apparatus for catalytic feeding release a five percent solution of the mixed catalyst into the measuring tanks, after which it enters the tank, where it is mixed with an organic solvent to the required concentration of 0.2%. From the tank, the finished catalyst mixture is discharged into the reactor, where it is maintained at the required pressure.

Ethylene is fed into the reactor from below, where it subsequently mixes with the catalyst and forms a working mixture. For the production of polyethylene under reduced pressure, contamination of the product with residues of the catalyst mixture is typical, which change its color to brown. Purification of the main product is carried out by heating the mixture, resulting in the destruction of the catalyst, further separation of impurities and their direct filtration from polyethylene.

The moistened product is sent to the dryer drying chambers bunker, where it is completely cleaned on a nitrogen fluidized bed (T = 373 K). Dry powder is poured from the bunker to the pneumatic line, where it is sent for granulation. Dust with polyethylene particles, remaining after nitrogen purification, is sent to the same line.

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Introduction

The average increase in PE consumption in Ukraine over the past 3 years amounted to 31% of all types of polymers. Current production of polyethylene in Ukraine is concentrated at CJSC "Lukor" (Kalush, Ivano-Frankivsk region). Annually this enterprise produces 70 thousand tons of polyethylene. This shows that such a product as polyethylene is relevant and consumed today. The main part of the produced polyethylene (50-60%) is used in the production of films and sheets. The remainder goes to injection molded products, coatings, insulating materials for the cable industry, extrusion products, blown products and pipes. But this is only a superficial overview of the use of polyethylene, which will be discussed in more detail in this paper.

This term paper is:

review and analysis of scientific and technical literature, gaining skills to work with it;

· study of the main material and technical processes of low-density polyethylene production technology;

Consideration of the raw material base from which it is made, including all kinds of additives that are added to polyethylene;

· study of the range of polyethylene, the use of products made from it and analysis of the position of polyethylene in the modern Ukrainian market;

· Consideration of the main methods for assessing the quality of polyethylene.

1. Assortment

High-pressure polyethylene (LDPE) is a solid elastic substance of a matte or pearly white color, resembling paraffin to the touch; it is odorless, non-toxic, combustible (continues to burn after being removed from the flame). Polyethylene produced at high pressure has a branched structure and low density Polyethylene belongs to the group of thermoplastic polymers. On fig. 1 shows a polyethylene granule.

Rice. 1 LDPE granule

Polyethylene is used when applying polyethylene insulation and sheathing to electrical cables. It is possible to extrude polyethylene in a mixture with powdered substances to obtain porous polyethylene.

Due to chemical inertness, lightness and strength, polyethylene bottles can store strong chemicals (sulfuric acid, hydrofluoric acid, etc.), as well as food products (milk, fats, juices), perfumes, medicines.

Pipelines made of polyethylene are much lighter and cheaper. Pipes are made with a diameter of 0.012-0.15 m and up to 1-1.5 m. The length of the pipes can reach 120 m. The flexibility and elasticity of the pipes allows them to be wound on drums, which is very convenient when transporting and laying them. Polyethylene pipes are absolutely not subject to corrosion, they do not burst when water freezes in them. IN chemical industry used in the transport of corrosive liquids. Fittings, valves, valves, linings and other connecting fittings are also made of polyethylene.

Polyethylene covers wood, paper, cardboard. It is applied from hot melts of polyethylene to paper and gives it gloss, print shine, good flexibility at low temperatures. Polyethylene is used to produce fibers that can be used to make marine ropes, filter nets, fabrics, upholstery fabrics for cars. IN textile industry polyethylene is used to impregnate fabrics in order to create a water-repellent material, improve tear resistance, and increase the strength of seams.

Medical instruments are made of polyethylene, it is used in plastic surgery and prosthetic technology.

The main injection molding is not only for individual machine parts, but also for housings for instruments and other figured products.

A significant part of the produced polyethylene (about 50%) is processed into films with a thickness of 0.01-0.1 mm, used as packaging material for storing easily moistened or, conversely, drying substances, such as fertilizers, cotton, silica gel, food products(meat, fish, bread, salt, flour, coffee, vegetables, fruits, etc.), as well as various products, apparatus, tools in order to protect them from corrosion.

Due to its excellent electrical insulating properties, polyethylene has become an indispensable material for insulating television, telephone and telegraph cables.

The addition of low molecular weight polyethylene to inks, varnishes and paints gives them increased resistance to abrasion. In the rubber industry, polyethylene is widely used as a lubricant that is perfectly compatible with various types of rubber.

Polyethylene, as a marketable product, is produced in pure form and with additives (various thermal and light stabilizers, additives against film sticking, etc.). They are introduced into polyethylene during processing in small quantities (tenths of a percent). Additives improve the quality of the finished polyethylene.

So, in the cable industry, polyethylene containing 0.5 and 2% soot is used. Polyethylene used for the manufacture of pipes for drinking and domestic water supply contains 2% soot (carbon black), and for drainage pipes up to 35% soot. Polyethylene when filled with talc, chalk, kaolin and other substances (up to 30-40% by weight) are used as structural materials for the production of sewer and drainage pipes, non-corrosive and fire-resistant fittings, as well as for cultural and household products, toys, dishes, etc.

Depending on the properties and purpose, polyethylene is produced in various grades indicated in Table 1.

Table 1. Grades of polyethylene, their areas of application and processing method

Application area

Processing method

Wire and cable insulation, cable sheaths

Technical products

Pipes and fittings for them:

pressure pipes

pipes non-pressure fittings

Films and film products:

special purpose

general purpose (technical products, films for Agriculture and etc.)

for the manufacture of bags for fertilizers and other purposes in agriculture

for food packaging

molding products:

with good elastic properties

with glossy surface

general purpose

open type, food contact

general purpose

containers and bottles for disinfectants with high resistance

Filling components (for filling parts of electrical equipment)

Covering paper, fabric, etc.

Coating for food packaging

Extrusion

Pressing

Extrusion

blowing

Extrusion

10203-003 10103-002 10702-020 10403-003

10003-002 10303-003

10103-002 10403-003

10203-003 15303-003

10603-007 17603-006

10702-020 15602-008

10903-020 17902-017

16902-020 15802-020

10802-020 11303-040

11502-070 11602-070

10203-006 17702-010

17602-006 10603-007

10802-020 15802-020

10903-020 17702-020

12002-200 18202-055

11903-080 12203-200

12103-200 12303-200

10702-020 11303-040

11102-020 11502-070

10702-020 11303-040

11702-010 18109-035

17902-017 11303-040

10203-003 11502-070

12402-700 16802-070

12502-200 18302-120

11502-070 16802-070

11802-070 18302-120

11502-070 16802-070

The designation of the basic grades consists of the name of the material "polyethylene" and eight digits. The first digit "1" indicates that the ethylene polymerization process proceeds at high pressure in tubular and stirred reactors in the presence of a catalyst. The next two digits indicate the serial number of the base brand. The fifth digit conditionally determines the density group of the polyethylene brand. The next three digits, written with a hyphen, indicate ten times the value of the melt flow index.

After the brand of polyethylene, the grade is indicated.

2. Feedstock for polyethylene production

2.1 Main raw material

Ethylene. Ethylene is a chemical compound described by the formula C2H4, a colorless gas with a slight odor. It is the simplest alkene (olefin). Contains a double bond and therefore belongs to unsaturated compounds, has a high reactivity. Ethylene is practically not found in nature. In small quantities, it is formed in the tissues of plants and animals as an intermediate product of metabolism. Plays an extremely important role in the industry, the most produced organic compound in the world.

At present, the main source of ethylene production is the pyrolysis of gaseous and liquid saturated hydrocarbons: ethane, propane, and straight-run gasolines.

Ethylene properties:

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

The 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 to ethylene are divided into two main groups - inert and active. An inert impurity, present in a noticeable amount, for example 5-10%, reduces the concentration of ethylene by a significant amount, given the low compressibility of ethylene.

Ethylene active impurities, 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 content of impurities, the specifications provide for the production of three grades of liquefied ethylene: A, B, and C. Ethylene of grades A and B is used for the production of polyethylene and ethylene oxide. Ethylene grade B - for the production of other organic products. Ethylene liquefied must comply with the requirements and standards.

Catalysts (initiators). As catalysts for the polymerization of ethylene, mainly molecular oxygen and organic peroxides are used. Of the peroxides in industry, peroxide of di-tert-butyl, tert-butylperbenzoate, etc., has found the greatest use. The effect of the initiator depends on the degree and rate of its decomposition at a given temperature and on the ability of the formed 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 (С8Н18О2)

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-butylperbenzoate (С11Н14О3)

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).

2.2 Auxiliary raw materials

Fillers - mainly solid inorganic or organic substances of natural (mineral and vegetable) and synthetic origin, which are introduced into the plastic mass to give it the appropriate properties.

Fillers are added to improve the properties of polyethylene (physical-mechanical, thermophysical, electrophysical, optical, aesthetic, technological, etc.). And cheap fillers reduce the cost of polyethylene, for example, when recycling polymers and plastics that are used as fillers.

The main types of fillers, as well as the properties they impart, are presented in Table 2.

Table 2. Examples of fillers with special properties

Composites

Filler examples

Abrasive

Antifriction

Biodegradable

highly flammable

Electrical insulating

electrically conductive

aesthetic

Sound and heat insulating

Structural

Magnetic

non-combustible

self-extinguishing

heat resistant

Heat storage

Friction

Chemical resistant

BN, SiC, diamond, quartz, corundum

MoS2, NbSe2, TiSe2, WS2, WSe2, graphite

starch, chitosan

Al, Mg, nitrates, permanganates, gunpowder

Al2O3, asbestos, quartz, mica, glass, talc

Metals (Al, Bi, Cd, Cu, Fe, Ni, Sn, etc.) and their alloys, graphite

Wooden thyrsa, marble chips

Glass wool, polyamide fiber

Metal and ceramic ferrite powders

Al(OH)3, Ca(OH)2, Mg(OH)2, sodium and zinc borates

Asbestos, graphite, carbon fibers

Wax, stearic acid, paraffin, glass spheres

BaSO4, asbestos

Asbestos, graphite, polytetrafluoroethylene, talc, technical coal.

Plasticizers are low-volatile, mostly liquid substances that give the mixture increased plasticity, as a result of which the molding of products is facilitated, the brittleness of the material at low temperatures is prevented, and its flexibility and elasticity increase. With an increase in the content of the plasticizer, the tensile and compressive strength of the polymer decreases, but the impact strength and elongation ability sharply increase. The most common plasticizers are butyl rubber, dibutyl phthalate, tricresyl phosphate, camphor, aluminum stearate, oleic acid, glycerin, etc.

Dyes are used to give the product the desired color.

Hardeners (for example, urotropine, lime, magnesia) are introduced into the composition of the plastic mass to accelerate the transition of the polymer to a solid, infusible state in which they do not melt or dissolve. In this case, the polymer forms a three-dimensional structure.

Stabilizers contribute to slowing down the aging process and, as a result, to the long-term preservation of polyethylene of its original properties. Stabilizers do not affect the initial properties of polyethylene.

Pore formers - for the production of foam and foam polyethylene.

Binders bind other components of the mixture into a monolithic material and determine the basic properties of the polymer. Synthetic resins are often used as binders.

Lubricants make it possible to improve the physical and mechanical properties of polyethylene, namely, to increase the homogeneity of the melt, increase its fluidity and relative elongation at break. Stearic acid, zinc oxide, barium stearate, etc. are added to the plastic mass as lubricants.

3. Polyethylene production

3.1 Theoretical basis ethylene polymerization process

Polymerization of ethylene at high pressure proceeds according to the radical chain mechanism, which consists of the stages of initiation, chain growth, and chain termination.

The initiation of the process consists in the formation of active radicals

The beginning of the reaction is the addition of ethylene to the formed radical, resulting in the formation of a new radical:

*CH3 + CH2=CH2 > CH3 -CH2-CH2*

Ethylene molecules are added sequentially to the radical formed by the reaction (growth reaction):

CH3 -CH2-CH2* + CH2=CH2 > CH3 -CH2-CH2-CH2-CH2*

Chain growth ends with a chain break. This usually happens when one inactive macromolecule is formed from two growing radicals:

CH3-CH2* + CH3-CH2* > CH3-CH2-CH2-CH3

Or, when two growing radicals form two inactive macromolecules, one of which has a double bond at the end:

CH3-(CH2-CH2)n-CH2* + CH3-(CH2-CH2)m-CH2* >

CH3-(CH2-CH2)n-1-CH=CH2 + CH3-(CH2-CH2)m-CH2*

These reactions reduce the rate of the polymerization process.

In the polymerization of ethylene according to the above mechanism, the formation of a linear saturated polymer should be expected.

However, in reality, depending on the reaction conditions, more or less branched macromolecules are obtained containing a small amount of double bonds (which is also due to the chain transfer reaction).

There are two variants of the chain transfer reaction on the polymer: intramolecular and intermolecular.

During intramolecular chain transfer from a growing polymer radical, one hydrogen atom is transferred from the secondary carbon to the end of the chain:

The secondary radical formed as a result of intramolecular transfer gives rise to the growth of a new side chain. The end section of the chain formed as a result of the transfer is a branching in the form of a side butyl branch. Thus, short side chains are formed. Branching in the form of long chains occurs as a result of intermolecular hydrogen transfer:

R1-CH2-CH2* + R2-CH2-CH2-CH3 > R1-CH2-CH2* + R2-CH*-CH2-CH3

3.2 Equipment for the production of polyethylene at high pressure

Polymerization of ethylene at high pressure is carried out in tubular or autoclave type reactors.

Polymerization can take place block way("in bulk"), when highly purified ethylene, compressed to a pressure of 100-300 MPa, is introduced into the reactor simultaneously with the process initiators, or in solution, when the reaction is carried out in a solvent medium.

Block polymerization is relatively difficult to control due to the high exothermicity of the process.

During polymerization, the reaction temperature as well as the viscosity of the reaction mass must be precisely controlled in order to improve mass transfer.

Heat removal through the reactor wall, cooling of the reaction mixture with fresh gas by partial additional injection into the reactor, lowering the temperature supplied to the polymerization of ethylene - all these measures do not provide sufficient heat removal to ensure that ethylene is polymerized by 100%. In order to prevent a large heat release, at which thermal decomposition of ethylene occurs, the reaction is artificially retarded at the stage corresponding to a 15-20% degree of conversion (30% at best). Unreacted ethylene is separated and recycled. Thus, the principles underlying the polymerization of ethylene at high pressure are quite simple, but the process is specific and requires sophisticated equipment, instrumentation, and automation.

3.3 The main technological scheme of an industrial plant

Technology system polyethylene production using liquefied ethylene is shown in fig. 2

The technological scheme of polyethylene production considered below is carried out in one stage, when all material flows move continuously along one thread, including the continuous processing of the polymer into commercial polyethylene.

Fresh ethylene of high purity, having passed the flowmeter 1 and the gas analyzer 2, is compressed by the reciprocating compressor 3, while its density reaches the density of light liquid hydrocarbons (400-500 kg/m3), and is sent through the aftercooler 4 to the ethylene condensation device 5, from where, together with the recycle gas, it enters the storage 6 of liquefied fresh and return ethylene.

Liquefied ethylene is taken from the storage and sent to the propylene refrigeration unit for "hypercooling". Subcooled ethylene is fed to a multistage centrifugal pump 7, in which it is compressed to an intermediate pressure - suction pressure of high pressure pumps. Before entering the high pressure system, ethylene is passed through a series of filters that remove impurities. Into the suction pipe by a high-pressure pump

pressure, additives, catalysts and air are introduced (with oxygen initiation). Ethylene containing additives and a catalyst enters a common manifold that feeds four identical high-pressure pumps 8 operating in parallel. Ethylene is compressed to a limiting pressure of 150-270 MPa. Ethylene after compression in high pressure pumps is fed into the reactor 9 at one or more points (200°C). At the outlet of the pumps and at the outlet to the reactor, the pressure is measured with special tensiometers. They show and record pressure. To automatically release ethylene into the atmosphere in the event of an increase in pressure above the set value, an emergency release valve is installed.

The reactor consists of a series of long, horizontal high-pressure pipes fitted with water jackets. These pipes have a very high length to diameter ratio. When the set temperature in the reactor is exceeded, the valve system is automatically activated to accelerate heat removal, which practically eliminates the possibility of thermal decomposition of ethylene.

The separation of the obtained polyethylene from unreacted ethylene is carried out in a large vertical polymer collector with a steam jacket 10. The polymer level in the apparatus is controlled and regulated by a special level gauge with a radioactive element.

Molten polyethylene from the collection enters the extruder 11 and is passed through a granulator filled with water. The resulting suspension of granules and water is directed to a 12 sieve and then to a 13 centrifugal dryer. The dried polymer flows by gravity into one of two hoppers.

From the product collector, hot gas, passing through the waste heat boiler 14, is cooled in a water cooler 15. Separation from low-molecular polymers is carried out in separators 16. Purified in traps filled with glass wool 17, the gas enters the column, in which oil and additives are separated from it. After liquefaction, ethylene 5 is sent to storage 6. The regenerated additives from the column are fed to the high-pressure pump 8 for mixing with ethylene.

There are various methods to improve the efficiency of polyethylene production. It should be carried out by introducing units of large unit capacity and intensifying production based on scientific and technological progress. Increasing the productivity of reactors due to the intensification and increase in the efficiency of their operation does not require large capital costs and is carried out by improving the design of reaction devices and optimizing the technological progress of polymerization.

An effective increase in the productivity of a unit of reaction volume is possible by increasing the conversion of ethylene per pass, which is mainly influenced by the following factors:

1) lowering the temperature of the gas entering the polymerization;

2) increasing the temperature in the reaction zone;

3) increasing pressure (to create a homogeneous reaction medium and increase the concentration of ethylene);

4) better removal of reaction heat, both due to better heat transfer through the wall, and due to better heat transfer through the wall, and due to a more perfect distribution of fresh gas along the length of the reactor;

5) Use of more efficient polymerization initiators;

6) Better mixing of the reaction mass;

7) Increasing the purity of the original ethylene;

8) Improvement of designs of reaction devices and technological schemes.

It is also interesting to recycle and recycle waste polyethylene, such as containers. Polyethylene packaging is used in many industries: cosmetic, chemical, food, etc. For reuse polyethylene, containers, from under different products, it is necessary to crush, dry, melt under vacuum and granulate. However, such polyethylene has a lower relative stretch index, i.e. it is less durable, and its composition is less homogeneous. These shortcomings are eliminated by adding lubricants to it.

4. Polyethylene quality control

4.1 Polyethylene quality indicators

production polyethylene assortment market

Polyethylene quality control is carried out both during the production of the material (in the reactor, at the outlet of the reactor, in the extruder-granulator), and in the laboratory already finished product. The quality of polyethylene is evaluated according to the following indicators:

Density;

· Molecular mass;

· Melt flow index;

· Viscosity;

· Scatter of melt flow rates within the batch;

The number of inclusions;

Technological test for appearance films;

· Resistance to cracking;

Tensile yield strength;

· Tensile strength;

· Elongation at break;

· Mass fraction extractables;

Smell and taste of water extracts;

· Resistance to thermo-oxidative aging;

· Resistance to photo-oxidative aging (by irradiation, according to the mass fraction of soot, according to the uniformity of soot distribution);

Mass fraction of volatile substances.

The main, of the listed indicators, according to which mandatory control quality, are the molecular weight of polyethylene, its density, viscosity, melt flow index. Table 3 presents quality performance standards for several basic grades.

Table 3 Quality indicators of basic grades of polyethylene

Name of indicator	Norm for the brand

1. Density, g/cm
2. Melt flow index (nominal value) with tolerance, %, g/10 min
3. Spread of melt flow rates within the batch, %, not more than: Top grade 1st grade 2nd grade
4. Number of inclusions, pcs., not more than: Top grade 1st grade 2nd grade
5. Technological test for the appearance of the film: Top grade 1st grade 2nd grade
6. Crack resistance, h, not less
7. Tensile yield strength, Pa (kgf/cm), not less than
8. Tensile strength, Pa (kgf/cm), not less than
9. Elongation at break, %, not less than
10. Mass fraction of extractable substances, %, not more than: premium 1st and 2nd grade
11. Smell and taste of water extracts, score, not higher
12 Resistance to thermal oxidative aging, h, not less
13. Resistance to photooxidative aging: irradiation method h, not less than: by mass fraction of soot, % according to the uniformity of soot distribution
14. Mass fraction of volatile substances, %, not more than: Top grade 1st and 2nd grade

4.2 Methods for determining quality

Molecular weight determination:

Polyethylene has a linear structure and can be dissolved in suitable solvents.

The molecular weight of linear polymers lies in the range of 103–107, and the polyethylene macromolecules formed during polymerization have different molecular weights, so polyethylene solutions are polydisperse systems, and the experimentally determined molecular weight is only an average statistical value.

The molecular weight of the crosslinked polyethylene fractions can be very large. It is determined by the degree of crosslinking, i.e. the average "molecular weight" between the crosslinking sites. The degree of crosslinking can be estimated from the degree of swelling of the polymer in solvents.

The molecular weight of polymers can be determined various methods, and each method is applicable to the measurement of molecular weights lying in the respective intervals.

All these methods, with the exception of the "end group" method, are based on a change in some properties of dilute polymer solutions in proportion to the number of molecules of the solute; complex apparatus is required to determine the molecular weight by such methods. Therefore, factories have so far usually used the simplest and fastest viscometric method and the molecular weight is calculated from the found value of the viscosity of the solution.

Method for determination of end groups. If there are functional groups at the ends of the macromolecule that can be determined chemically, then based on the chemical analysis data, the number average molecular weight of the polymer can be calculated. Since the relative number of end groups in a polymer sample with a high molecular weight is very small, the accuracy of their determination is low. This method determines the molecular weight up to 3 104.

Ebullioscopy and cryoscopy. In these methods, molecular weight is calculated from the increase in boiling point or decrease in freezing point of polymer solutions. Since the temperature changes here are very small, the accuracy of these methods is also low.

When using the ebullioscopic method, a solvent with a low boiling point is used to avoid degradation of the polymer. The choice of solvent for the cryoscopic method is even more difficult, since. how polymer macromolecules can precipitate out of the solvent before reaching the freezing point of the solvent or together with the solvent. The interval for determining the molecular weight is 2·104-3·104.

Osmotic pressure method. When using this method, significant difficulties arise in the manufacture of semipermeable membranes capable of passing solvent molecules and retaining macromolecules with a molecular weight of up to 30,000 (the use of the osmotic method for polymers with a lower mass is not reliable). The interval for determining the molecular weight is 104-106.

Light scattering method. A light beam passing through a transparent medium is partially scattered. The method is based on the fact that a pure solvent and a polymer solution have different degrees of light scattering. The resulting molecular weight is the weight average molecular weight. The interval for determining the molecular weight is 104-107.

Settling (or sedimentation) method in an ultracentrifuge. When settling a suspension, the gradual settling of particles and the settling rate can be used to calculate the mass of particles of the suspended substance, if a very strong centrifugal field is used, in an ultracentrifuge. The rotational speed of the centrifuge rotor must be at least 1000 rpm. From the rate of deposition, one can calculate not only the molecular weight of the polymer, but also the distribution over molecular weights. The interval for determining molecular weights is 104-107.

Viscometry method. The simplest and most convenient method for determining the molecular weight is the viscometric method. Molecular weight is calculated from an empirical equation relating solution viscosity, solvent viscosity, and polymer concentration. The molecular weight calculated from the viscosity characteristic is called the viscosity average molecular weight and is usually expressed by the value of its logarithm.

Determination of the melt flow rate: the apparatus for determining the MFR (GOST 11645--73) is a syringe plastomer, the inner diameter of the nozzle of which is 2.09 mm, with a rod and a weight on it equal to 2.16 kg, a thermocouple for measuring the melt temperature, which, when determining the index, is maintained constant at 463 K ± 0.5 (190 ± 0.5 ° C). The mass of material in grams extruded for 10 minutes under these conditions is called the melt flow index. A low melt index corresponds to the high internal friction inherent in a high molecular weight material. Thus, the melt flow rate determined by this method makes it possible, with a known approximation due to insufficient measurement accuracy, to classify polyethylene grades according to the size of polymer molecules.

Determination of apparent density (bulk mass):

Method of measurement and weighing. The method consists in determining the density of a substance by the ratio of the mass of the sample to its volume, determined by direct weighing and measurement. It is possible to measure the volume by other methods, such as the displaced volume of liquid for samples of irregular or difficult to measure shape. The method is used to determine the density (volume weight) of products and semi-finished products (rods, bars, pipes) and provides a measurement accuracy of up to 0.5% with an accuracy of 0.3% volume measurement and 0.2% mass.

Hydrostatic weighing method. The method consists in comparing the masses of equal volumes of the test substance and a liquid of known density (eg distilled water). The method is designed to determine the density (bulk weight) of molded products (rods, bars, tubes); it provides measurement accuracy up to 0.1%.

Pycnometric method. The method consists in comparing the masses of equal volumes of the test substance and a liquid of known density. The method is used to determine the density of molded products, press-powder granules, flakes; it provides measurement accuracy up to 0.05%.

The flotation method consists in comparing the density of the sample with the density of a known liquid at the moment the sample enters a suspended state. The method is used to determine the density of plastics (mainly polyolefins) in the form of granules and any molded products. A mixture of ethyl alcohol and water is used as a working fluid. The method is suitable for determining the density of polymers from 910 kg/m3 (0.9100 g/cm3) with an accuracy of 0.0002 g/cm3.

The gradient column method is based on comparing the immersion depth of the test sample and a liquid of known density in a cylinder or tube with a solution whose density varies with height ("gradient column").

The method is used to determine the density of products in the form of films, granules, fibers, as well as any molded products. The accuracy of this method depends on the density difference of the liquid along the height of the gradient column. With a column "sensitivity" of 0.0001 c/cm 3 per millimeter, the accuracy of the method reaches 0.05%.

Currently, polyethylene, both low and high density, is widely distributed on the market, the bulk of which falls on containers and packaging. various kinds products. Therefore, it is necessary to pay a lot of attention to the quality and properties of this material.

In the course of the work done, I learned that high-pressure polyethylene has a low density and belongs to the group of thermoplastic polymers. It has chemical inertness, lightness and strength, the ability to stretch. Such qualities have determined the scope of its application, where polyethylene is used in the form of films, packaging material, anti-corrosion coatings, electrical insulating materials for cables, they are impregnated with fabric and paper.

The raw material for polyethylene is ethylene and catalysts. But in its pure form, it is rarely produced. The variety of its brands is explained by the introduction of additives into polyethylene, such as fillers, plasticizers, binders, hardeners, dyes, stabilizers, lubricants. Additives give polyethylene certain specific properties and improve its quality.

I also learned that the polymerization of polyethylene takes place at elevated temperatures and pressures, and in order to prevent thermal decomposition of ethylene or inhibition of the reaction, constant monitoring is needed. Therefore, a large number of instrumentation and automation are used in production.

The main indicators by which polyethylene is characterized are its molecular weight, density and melt flow. According to these indicators, the quality of polyethylene is determined in laboratories, as well as in the production itself: in the reactor, directly at the exit from the reactor, ready-made polyethylene granules.

Polyethylene technology requires strict adherence to production regulations, taking into account the impact technological parameters on the properties of the finished product, strictly organizational process. Only with this approach can you get high-quality material.

The recycling of waste polyethylene has become an unusually topical topic today, since it does not decompose and pollutes environment. Scientists have already developed several methods for recycling polyethylene, which is possible due to its thermoplastic properties. However, the difficulty is the need for powerful equipment and waste sorting.

Bibliography

1. Shifrina V., Statsky N. High pressure polyethylene. Reference guide - Gostkhimizdat, 1975 - p. 45-50.

3. Kavarnovsky S.N., Kozlov V.N. Technological schemes of the processes of basic organic synthesis. Methods for the production of initial products of macromolecular compounds. K .: Gorky, 1968 - p. 122-124.

4. T.M. Tomilina, L.M. Zabolotnikova, V.V. Vakush, I.A. Mochalnik, N.P. Grishin. Fundamentals of technology of the most important industries: in 2 hours. Part 2: Proc. Allowance for universities; Ed. I.V. Chentsova, V.V. Vashuka. - Mn.: Vysh. school., 1989 - p. 79

5. Yu. Kovalyov. Overview of the Ukrainian polyethylene market. Magazine "Polymers-money". Ed. V. Kuzovenko. - 2006 No. 8 - p. 19-22.

6. O.P. Mantulo, I.M. Novikov. Polymeric containers with PET are pressed in, processing paths. Journal "Chemistry Industry of Ukraine" Ed. Yu.M. Sidorenko - 2006 No. 1 - p. 51-53.

7. I.O. Mikulyonok. Thermoplastic composite materials and their refills, classification and heat shields. Journal "Chemistry Industry of Ukraine" Ed. Yu.M. Sidorenko - 2005 No. 5 - p. 30-39.

8. GOST 16337-77 High pressure polyethylene. Specifications. Introduction 01/01/1979 - M .: IPK Standards Publishing House - 1979 - p. 70

9. GOST 11645-73 Plastics. Methods for determining the fluidity index of a thermoplastic melt. Introduction 01/01/1975 - M .: Standards Publishing House. 1975 - p. 12

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Types of raw materials for the production of bags

High pressure polyethylene. This raw material for the production of bags appeared in the 30s of the last century. Easy to manufacture, LDPE is elastic, tensile strength, water and gas impermeable. However, it is sensitive to oxidation, and therefore cannot be used for the manufacture of food packaging (films). We use high-density polyethylene in the production of garbage bags, T-shirt bags, packaging products.

Low-pressure polyethylene. HDPE is more difficult to manufacture, and it only began to be made in the 50s. Accordingly, low-pressure polyethylene products came into use much later. The main advantages of HDPE - high resistance to aggressive environments various types. However, HDPE is less resistant to water, low temperatures, and gas. Such polyethylene can pass liquid and gas. HDPE bags are made for wine, shrink stretch, bags.

Resale. Secondary raw materials for the production of bags are as important as HDPE and LDPE pellets. At our enterprise it is one of the most common types of raw materials. Using the secondary, we do not just save own funds, while producing equally high-quality products, but we also rely on improving the environmental situation in the country. We recycle used material that would otherwise be incinerated or buried in a landfill. Bags made from recycled polyethylene can be recycled a third time. However, the number of revisions is limited.

We use the best raw materials for the production of bags!

"KSK-Supply" is a company where you can buy high-strength packages. We carefully monitor the production itself and the raw materials for the production of bags. Only best materials, corresponding to the standards, fall into the workshops of the enterprise. By relying on the quality of raw materials, we can guarantee the quality of the final product.

Polyethylene is the cheapest non-polar synthetic polymer, which belongs to the class of polyolefins. Polyethylene is a white solid with a grayish tint.

The first to study the polymerization of ethylene was the Russian chemist Butlerov in 1873. But an attempt to implement it was made in 1884 by the organic chemist Gustavson.

Polyethylene production technology + video how they do it

Everyone is engaged in the production of polyethylene large companies petrochemical industry. The main raw material from which polyethylene is obtained is ethylene. Production is carried out at low, medium and high pressures. As a rule, it is produced in granules, which have a diameter of 2 to 5 millimeters, sometimes in the form of a powder. To date, four main methods for the production of polyethylene are known. As a result, we obtain: high-pressure polyethylene, low-pressure polyethylene, medium-pressure polyethylene, as well as linear high-pressure polyethylene. Let's look at how the production of MPE is carried out.

High pressure polyethylene is formed at high pressure as a result of the polymerization of ethylene in an autoclave or in a tubular reactor. Polymerization in the reactor is carried out by a radical mechanism under the influence of oxygen, organic peroxides, they are lauryl, benzoyl, or mixtures thereof. Ethylene is mixed with an initiator, then heated to 700 degrees and compressed by a compressor to 25 megapascals. After that, it enters the first part of the reactor, in which it is heated to 1800 degrees, and then into the second part of the reactor for polymerization, which occurs at a temperature in the range from 190 to 300 degrees and a pressure of 130 to 250 megapascals. In total, ethylene is in the reactor for no more than 100 seconds. The degree of its conversion is 25 percent. It depends on the type and quantity of the initiator. The ethylene that has not reacted is removed from the resulting polyethylene, after which the product is cooled and packaged.

LDPE is produced in the form of both undyed and colored granules. The production of low-density polyethylene is carried out according to three main technologies. The first is polymerization, which occurs in suspension. The second is polymerization occurring in solution. This solution is hexane. The third is gas-phase polymerization. The most common method is polymerization in solution. Solution polymerization is carried out in the temperature range from 160 to 2500 degrees and pressure from 3.4 to 5.3 megapascals. Contact with the catalyst is carried out for about 10-15 minutes. Polyethylene is separated from the solution by removing the solvent. First of all, in the evaporator, and then in the separator and in the vacuum chamber of the granulator. Granular polyethylene is steamed with water vapor.

HDPE is produced in the form of both undyed and colored granules, and sometimes in powder form. The production of medium pressure polyethylene is carried out as a result of ethylene polymerization in solution. Medium pressure polyethylene is obtained at a temperature of approximately 150 degrees, a pressure of not more than 4 megapascals, and also in the presence of a catalyst. PSD from the solution precipitates in the form of flakes. The product obtained as described above has a weight average molecular weight of not more than 400 thousand, a degree of crystallinity of not more than 90 percent. The production of linear high-pressure polyethylene is carried out using the chemical modification of LDPE. The process takes place at a temperature of 150 degrees and approximately 30-40 atmospheres. Linear low density polyethylene is similar in structure to high density polyethylene, however, it differs in longer and more numerous side branches. The production of linear polyethylene is carried out in two ways: the first is gas-phase polymerization, the second is polymerization in liquid phase. She is currently the most popular. As for the production of linear polyethylene by the second method, it is carried out in a fluidized bed reactor. Ethylene is fed into the reactor, while the polymer is in turn withdrawn continuously. However, the level of the liquefied bed is constantly maintained in the reactor. The process takes place at a temperature of about one hundred degrees, pressure from 689 to 2068 kN/m2. The efficiency of this method of polymerization in the liquid phase is lower than that of the gas phase.

Video how to do it:

It is worth noting that this method also has its advantages, namely: the size of the installation is much smaller than that of equipment for gas-phase polymerization, and much lower capital investment. Practically similar is the method in the reactor with a stirrer using Ziegler catalysts. This results in maximum output. Not so long ago, for the production of linear polyethylene, technology began to be used, as a result of which metallocene catalysts are used. This technology makes it possible to obtain a higher molecular weight of the polymer, thereby increasing the strength of the product. LDPE, HDPE, PSD and LDPE differ from each other, both in their structure and properties, respectively, and they are used to solve various problems. In addition to the above methods of ethylene polymerization, there are others, but they have not received distribution in industry.

The main industrial method for the production of LDPE is the free radical polymerization of ethylene in bulk at a temperature of 200-320 °C and pressures of 150-350 MPa. Polymerization is carried out on continuous operation units of various capacities from 0.5 to 20 t/h.

The technological process of LDPE production includes the following main stages: ethylene compression to the reaction pressure; indicator dosing; modifier dosing; polymerization of ethylene; 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 to analysis bins and determination of polyethylene quality, formation of batches in commodity bins, mixing, storage; loading polyethylene into tanks and containers; packaging in bags; additional processing - obtaining compositions of polyethylene with stabilizers, dyes, fillers and other additives.

2.1. TECHNOLOGICAL SCHEMES.

LDPE production facilities consist of synthesis units and confection and post-treatment units.

Ethylene from the gas separation unit or storage is supplied under a pressure of 1-2 MPa and at a temperature of 10-40 °C to the receiver, where return low-pressure ethylene and oxygen (when used as an initiator) are introduced into it. The mixture is compressed by an intermediate pressure compressor up to 25-30 MPa. is connected to the return ethylene flow of intermediate pressure, compressed by the 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 by means of a pump directly before the reactor. Ethylene is polymerized in the reactor 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 to 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, the separation of polyethylene and unreacted ethylene occurs. Molten 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 takes place and low molecular weight polyethylene is released, and then it is fed 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 purification system (refrigerator, cyclone). From the receiver, the low-pressure return gas compressed by the 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 confectioning and additional processing.

It is possible to obtain some compositions in the primary granulation extruder. In this case, the extruder is equipped with additional units for introducing liquid or solid additives.

A number of additional nodes in comparison with the technological scheme for the synthesis of traditional LDPE has a technological scheme for the production of linear high-pressure polyethylene, which is a copolymer of ethylene with a higher a-olefin (butene-1, hexene-1, octene-1) and obtained by copolymerization according to the anion-coordination mechanism under the influence of complex organometallic catalysts. Thus, the ethylene supplied to the plant undergoes additional purification. Comonomer - a-olefin is introduced into the return gas of intermediate pressure after its cooling and purification. After the reactor, a deactivator is added to prevent polymerization from occurring in the polymer-monomer separation system. Catalysts are fed directly into the reactor.

In recent years, a number of foreign LDPE manufacturers have organized the production of LLDPE at industrial LDPE plants, equipping them with the necessary additional equipment.

Granular polyethylene from the synthesis unit, mixed with water, will be fed to the polyethylene dehydration and drying unit, which consists of a water separator and a centrifuge. Dried polyethylene enters the receiving hopper, and from it through automatic scales into one of the analysis bins. Analysis bins are designed to store polyethylene for the duration of the analysis and are filled one by one. After determining the properties, the polyethylene is sent by means of pneumatic transport to the air mixer, to the bunker of the substandard product or to the bunkers of the commercial product.

Polyethylene is averaged in an air mixer 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, tankers or containers, as well as for packaging in bags. All hoppers are purged with air to prevent ethylene accumulation.

To obtain compositions, polyethylene from the bunkers of the commercial product enters the supply bunker. Stabilizers, dyes or other additives are fed into 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, water separation in a water separator and drying in a centrifuge, the polyethylene composition enters the commercial product bins. From the bunkers, the product is sent for shipment or packaging.

Production of low pressure polyethylene. Delivery basis map

Raw material

High pressure polyethylene production technology

Process technology

Low pressure polyethylene production technology

Process technology

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