Pipes of different types of polyethylene

There are two modes of polyethylene production: to obtain polyethylene of low consistency under the high pressure and to obtain polyethylene of high and middle consistency under the low pressure. Usage of low consistency polyethylene for ascending pipes is limited by minor diameters and range of application requires high flexibility of pipes because its toughness is 2-3 times as less than toughness of high consistency polyethylene.

The first high consistency polyethylene used for ascending pipes producing was linear homopolymer the high molecular chain of which consists of only ethylene molecules. Under the high enough consistency during the short period of time homopolymer possessed low immunity to cracking and due to changing of destruction nature, i.e. transition from flexibility to brittleness, strengthening qualities were reduced dramatically during the long–term operation. MRS value defined the long-term strength and used for accounting of working pressure of pipelines were 6,3 MPa.

Intention to increase the immunity to cracking and to avoid transition from flexibility to brittle destruction within the period of operation led to production of polyethylene of second generation. Due to adding the somonumbers (butene and gexen) to the process of synthesis which form lateral branches on the polyethylene macromolecules increasing of the polyethylene immunity to cracking and increasing of MRS value up to 8,0 MPa were successful. However after that short-term strength, flexibility module and immunity to fast cracks spreading were decreased what makes impossible the using of such polyethylene in gas pipelines producing for the pressure much than 6 atmospheres.

Combination of high short-term strength and high immunity to cracking was made by creating of so-called bimodal polyethylene – polyethylene of third generation. At the expense of dedicated conduct of technological process (mostly according to double reactor scheme) usually there are obtained two pronounced groups of macromolecules – short- and longchain. And somonumber is entering to high molecular part of polymer which provides high immunity to cracking of polyethylene. Low molecular part of polymer forms crystalline areas at the expense of which density short and long term density (MRS 10,0 MPa) as well as strength module increase. Polymer possesses high immunity to fast cracks spreading and according to this indicator there are no obstacles for producing of gas pipes under the 12 atmospheres and water supply pipes under the 25 atmospheres.