The modern world is hard to imagine without plastic. Today, everything that surrounds us is one third composed of all sorts of plastics. We are so used to them that we do not always notice them in everyday life. At the same time, plastics are a profitable business that brings its owners millions of dollars a year. Among them, the most common and most profitable is the production of polyethylene.
Polyethylene can be divided into two main subspecies: high density polyethylene, obtained by low pressure, and low density polyethylene, obtained by high pressure. It is about PVD – high pressure polyethylene – and will be discussed in this article.
Scope of polyethylene is quite wide. For the most part, high-pressure polyethylene is used to release films and various filmy products: thermofilms, bags, multi-layer food packaging, greenhouse films. High-pressure polyethylene products are used in electrical engineering, chemical and food production, automotive, construction, etc. High-pressure polyethylene pipes have unmatched strength and are recommended for installation in residential communication systems.
WHAT IS A PVD?
LDPE (also known as LDPE, also known as LDPE) is an almost completely transparent and slightly plastic material that is not susceptible to chemical influences in a free state, an excellent electrical insulator, frost-resistant, not exposed to radiation, moisture-absorbing and gas-resistant.
If you speak a more scientific language, the technical characteristics of high-pressure polyethylene are as follows:
– melting point of high-pressure polyethylene from 105 to 115 degrees Celsius;
– density of high-pressure polyethylene from 917 kg / cubic meter to 935 kg / cubic meter;
– maximum operating temperatures from -45 to 70 degrees Celsius;
– tensile strength 10-17 MPa;
– elongation (relative) from 50 to 600%;
– hardness of 14-25 MPa;
The material for the production of polyethylene is unsaturated hydrocarbon – ethylene. In order for a gas to form a PVD, it must be polymerized, that is, to create covalent bonds between hydrocarbon molecules. However, this should be done under certain conditions: heat the base substance to a temperature of 200 to 260 degrees Celsius, compress the gas under a pressure of not less than 150 MPa (but not more than 300 MPa) and initiate the reaction using an activator. This activator-initiator can be oxygen, peroxide, or nitro compounds (i.e., containing NO2 nitrile).During heating, activator molecules break down into free radicals. These radicals react with CH2 (ethylene). Due to the heating and addition of the activator, the initiated ethylene molecule begins to actively take on more and more new CH2 molecules. In other words, ethylene polymerization begins. LDPE base stamps have a non-linear structure, with multiple side branches of the chain from the center of the onset of the reaction. Sometimes the side chain is equal to or even exceeds the length of the main chain.

LDPE TECHNOLOGY
Production of high-pressure polyethylene can follow one of three technologies: thermoplimerization in bulk, in solvent or in suspension – and in one of two types of polymer plants: tubular reactor or cylindrical, vertically arranged, equipped with a special mixing mechanism called an autoclave.
By the way, at the moment both types of installations for the polymerization of ethylene under high pressure are distributed about the same, since some brands of LDPE can be obtained only in one or another form of equipment.
Different installations produce polyethylene of different properties, which allows the manufacturer to change the parameters of the finished product at his own discretion by controlling one of the following indicators:
– system pressure;
– heating temperature;
– the amount of initiating substance;
– duration of exposure.
What will change? The greater the pressure, the greater the mass of the molecules and the smaller the lateral compartments in the structure of the PVD molecule. The more initiating substances, the opposite is less mass, faster reaction rate and more O2 molecules. The hotter, the less mass, the greater the branching from the center of the reaction and the faster the reaction itself. And lastly, the longer a substance is in the system, the faster it turns and the more its mass becomes.
However, everything has its own redistribution. For example, if you do not control the dosage of the substance-initiator, then at a certain ratio of activator / temperature, an explosion and decomposition of ethylene into atoms occurs with the release of methane. At the same time, if the temperature becomes significantly higher than the norm, and the heat removal system does not have time to cool the substance in time, an uncontrolled acceleration of the reaction can occur, and then an explosion with subsequent decomposition of ethylene into atoms.
Control over the quality of polyethylene is carried out by a special GOST (No. 16337-77 from 01.01.79). According to it, LDPE can be of two categories: basic (it means there are no additives in it) or composite (base + various additives, can be colorless or color).

GRADE POLYETHYLENE

The basic grades of polyethylene are of 2 types, depending on the type of device producing them (autoclave or tubular reactor) and 3 grades (top, first and second). If LDPE is produced in granular form, then all its particles must be of the same shape and size. The size limits for LDPE particles are normally from 0.2 to 0.5 cm. Also, small deviations from the norm are allowed in each batch (ie, the presence of particles larger than the base indicator), but it should be noted that their percentage should not exceed established by GOST limits.
In total, GOST establishes 8 base LDPE grades for autoclaves and 21 marks for tubular radiators. The digital code laid down in the name of each brand makes it easy to determine which product is in question (for LDPE, the first digit is always 1, it indicates the presence of high pressure in the system), on which equipment LDPE is produced (second and third digit in the brand name, number from 01 to 49 are assigned to LDPE produced on an autoclave, the numbers from 50 to 99 are LDPE from tubular reactors), as was the averaging of the polymer compound (0-cold mixing, 1 with melt), what density is the substance (there are 6 classes in total), and also indicator nditions LDPE (number after the dash).
At the moment, domestic producers of polyethylene are located in Angarsk, Tomsk and Kazan. In the CIS countries, LDPE is produced in Belarus (Novopolotsk). The most well-known foreign manufacturers of LDPE: TVK, Vordian, Polimeri Europa, NOVA Chemicals, Basell, etc.
Along with the classic forms of LDPE, in recent years, the release of a similar modification to it – linear polyethylene (LLDPE or LLDPE) has begun to gain momentum. Its structure is as linear as that of low-pressure polyethylene, but has a greater number of branches.