PEX pipe, also known as cross-linked polyethylene pipe, is made of polyethylene material, and the linear molecular structure of polyethylene is transformed into a three-dimensional network structure through physical and chemical methods, thereby improving the performance of polyethylene. According to the method of its manufacture, it is classified as follows:
The Engel method is a peroxide method in which a cross-linking medium peroxide is used to form a chemical bond between long molecular chains of polyethylene by high temperature and high pressure, so-called cross-linking. This cross-linking method is named PX-a; a silane method in which a silane is mixed into a raw material during extrusion. The polyethylene chain is combined with the silicon molecule and is classified as PX-b; the amount of antioxidant in the material is one of the most important factors affecting the life of the pipe; the radiation method: the polyethylene is irradiated with γ or β rays, thereby Physically formed cross-linking, called PX-c; azo method: cross-linking through azo medium, known as PX-d, is currently not traded.
Commercially available PEX tubes are currently the first three types. The performance of the three PEX tubes is not identical, mainly due to the difference in heat resistance (heat strength), creep resistance and stress crack resistance. In general, in a macromolecular structure, a large molecule of a two-dimensional network structure has a relatively simple thermal motion, and a large molecule of a three-dimensional structure has a slightly hot thermal motion. The macromolecules of PEXa are mainly two-dimensional network structures, while the macromolecules of PEXb and PEXc are mainly three-dimensional structures. Therefore, when the same polyethylene is used as the base material, PEXb and PEXc have higher heat resistance, creep resistance and stress crack resistance than PEXa when the degree of crosslinking is the same. Progressive PEXa cross-linking, their difference will be reduced. For example, the international standard ISO/DIS15875 and the advanced standard DIN16892 specify the degree of crosslinking of the three PEX tubes as follows: 1. PEXa ≥ 70%; 2. PEXb ≥ 65%; 3. PEXc ≥ 60%.
As long as the above degree of crosslinking is achieved, the heat resistance, creep resistance and stress crack resistance of the three PEX tubes are substantially the same. 20 °C 12Mpa ≥ 1h, 95 ° C 4.8Mpa ≥ 1h, 95 ° C 4.6Mpa ≥ 165h, 95 ° C 4.4Mpa ≥ 1000h, 110 ° C 2.5Mpa ≥ 8760h Although the basic performance of the three PEX tubes is consistent, but there are still some subtle The difference, this nuance gives the PEXa tube a unique application value. As a result of the superposition of the two-dimensional network structure, the interstices between the macromolecules are small, the void ratio is very low, and the macromolecules of the three-dimensional bulk structure have a "hollow" phenomenon, and the interstices between the macromolecules are large, and the void ratio is high. Therefore, the oxygen permeability of the PEXa tube is lower than that of PEXb and PEXc. It is also because of the two-dimensional network structure of PEXa that the flexibility of macromolecules is higher than that of PEXb and PEXc. The pipe is easier to bend during construction, and more importantly, the internal stress generated by the bent part is lower than that of PEXb. The service life impact is minimal.
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