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About Polyethylene

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The rapid development of plastic technology led to significant changes in the field of production of raw materials. As a whole, there are two methods of the production of polyethylene: low density polyethylene is produced under high pressure, medium and high-density  - under low pressure.

 

Low-density polyethylene is used for small diameter pipes of limited purpose as their durability is 2 or 3 times less than the durability of high-density polyethylene.

MRS (Minimum Required Strength) -  is a durability indicator identifying features of the material used in the production of long-lasting minimum durability pipes. That is, minimum reliability required for pipes at 20oC temperature under 10 atm working pressure. In other words, the minimum required pipe strength. The durability term of PE100 pipes is 5 years.

PE63 polyethylene obtained in 1950s was successfully used in systems which do not require high pressure. Due to its technical characteristics, use of this material was possible only for low-pressure systems (max 4 atm). The first high density polyethylene for pressurized pipes PE63 was a linear homopolymer of high molecular chain consisting only of ethylene molecule. The homopolymer had low resistance to cracking and used to undergo physical changes. That is? During long term use it transferred from plastic form to fragile form. The pipes had 6.3 atm MRS. According to this indicator polyethylene used in the production of this type of pipes was PE63.

Measures for the increase of cracking resistance during operation, and prevention of fragility showed their results. There was created second generation polyethylene. During the synthesis process application of somonomeres (butane or hexene) sharply increased resistance of polymer to cracking and increased working pressure up to 8.0 atm. Due to this characteristic, polyethylene was called PE80.

However, in this case the minimum term of durability, flexibility and cracking resistance decreases, which does not allow using of this polyethylene in the production of gas pipes designed for the pressure of 6 atm.

Joint conformance of high short-term durability and high cracking stability was obtained by the invention of the third generation polyethylene. In the result of a technologic process of double-reactor diagram two groups of short-chain and long-chain macromolecules were obtained. In this case, it is included into high molecular part of somonomer, and this provides maximum cracking resistance of polyethylene. Low-molecular part of polymer, forms a crystallic field, which provides increase of strength, short-and long-term durability (10 atm), and flexibility. According to this characteristic, polyethylene used in the manufacture of such type of pipes is called PE100.

Resistance of pipes against rapid spread of cracks is high, and therefore there is no limit for the production of gas pipes with 12 atm, and water pipes with up to 25 atm operating pressure.

PE100 polyethylene has high technical characteristics. In spite of the high melting adhesiveness, low molecular fraction of the alloy functions as lubrication, and facilitates extrusion of the polymer. Production of pipes does not pose a problem and modern equipment is not applied additional requirements.

Use of PE100 polyethylene in large diameter pipes allows double reduction of the material capacity. For example, while operating pressure of pipes made of PE80 raw material according to SDR11 is 10 atm, the pressure of pipes made of PE100 raw material is 16 atm. That is, pipes made of PE100 polyethylene have less wall thickness and higher pressure resistance.

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