Development Trend of High Density Polyethylene Water Supply Pipeline
1. Historical development trend of raw materials
The development trend of high-density polyethylene can be divided into three stages: the first stage from the 1950s to the 1970s, PE raw materials of grades below PE63 only considered the 50-year pressure capability [1]; the second stage From the early 1980s to the 1990s, PE80 grade materials, with no ductile failure within a 50-year service life, have achieved high environmental stress cracking performance and comprehensive flexibility and long-term durability by introducing copolymer units. Strength performance; third stage: Since the early 1990s, PE100 raw materials produced by copolymer processing have been developed and have good resistance to slow crack growth and rapid crack expansion. The needs of the market have been appropriately met in terms of 50-year service life, pressure-bearing capacity, resistance to rapid cracking, and resistance to slow cracking. At the same time, the pipe wall thickness has been reduced, which has greatly promoted the application of high-pressure polyethylene pipes in water supply and drainage. The application of radiant heating industry has brought the application of high-pressure polyethylene pipes in water supply and drainage radiant heating systems to a new level. In recent years, PE100-RC raw materials with better resistance to slow crack growth have been introduced, which have stronger resistance to slow crack growth and rapid crack propagation.
2. Development trends of raw materials and product performance
Under the influence of the continuous product research and development innovation of compounding ingredients manufacturing companies and industry application requirements, the production capacity and business scale of compounding ingredients manufacturing companies continue to develop, and the objective situation stipulates that relevant national industry standards should be upgraded and developed with the trend . The current national industry standards GB/T 13663.1 (General Principles), GB/T 13663.2 (Pipe Materials), GB/T 13663.3 (Pipe Materials), and GB/T 13663.5 (System Software Applicable Scope) published successively from 2017 to 2018 are formulated in accordance with ISO 4427 (Series: 2007 has undergone great changes, with stronger requirements for various performances. The European standard EN 1222 (series) has also been upgraded to 2010 [2,3,4]. For GB/T Analysis of the key performance requirements in 13663 and GB/T 13663-2000/GB/T13663.2-2005-ISO4427 (series): 2007, EN12201 (series): 2010:
1) Improve mixing ingredients and related regulations. Black compounding requires that the carbon black content in the epoxy resin be 2% to 2.5%. The purpose of adding carbon black is to color it, and on the other hand, it can shield the ultraviolet rays and reduce damage to the epoxy resin. In addition, the dispersion of carbon black/color paste has great harm to the physical properties of the mixture [5]. The dispersion is not very good, and the macroeconomic properties such as elongation at break and impact properties will be greatly reduced. In addition, carbon black is required. The particle size of black particles increases the grain size of the mixture, which is mainly reflected in the improvement of relative density and flexural modulus. A single material is required, not “white and black”. The screw extruder composition has a major impact on the performance of the product. If the composition is weak, the carbon black will not be dispersed well. If the composition is too strong, excessive shear stress will damage the molecular formula, which will cause This results in a reduction in the mechanical properties of the mix structure. Therefore, in order to ensure product quality, raw materials are required to be evaluated ingredients.
2) Long-term hydrostatic strength and minimum specified strength: The long-term hydrostatic strength of materials/tubes/tubes is the basis for calculation of key parameters such as shear strength, pressure level and wall thickness[1] , the long-term hydrostatic strength can be obtained according to GB/T 18252 (or ISO 9080), thereby obtaining the minimum specified strength MRS. MRS is the axillary stress that is continuously released on the inner wall of polyethylene water pipes for 50 years, causing pipe damage. Its experimental results are an important indicator of pipe performance. It is also the design theory of PE 80 and PE 100 grade materials and is relevant to the application under the 20°C standard. 50 years of geostress causing pipeline damage. National industry standards GB/T 13663.1-2018, ISO 4427-1:2007, and EN1222-1 all stipulate that the raw materials for drainage pipes are PE 80 and above grade compounds, and the PE 63 compounds required in GB/T13663-2000 are deleted.
3) Improved rapid crack propagation resistance (RCP). The accumulation of gas pressure in the pipe and at the same time subjected to extreme effects such as low temperature and severe impact, cause the pipe to crack at a very fast rate in the radial direction, a random damage method. Generally, the lower the temperature and the thicker the pipeline, the greater the risk of rapid cracking. It can be seen that the critical pressure PC size of the mixture is directly related to the size of the maximum pressure MOP of the pipeline system.
4) Improve resistance to slow cracking (SCG). The increase in slow-speed cracks of PE pipes is mainly caused by some high stress points in the pipes, such as small defects on the surface and pits caused by the construction site. Therefore, generally, materials above PE 80 grade generally have relatively high resistance to slow crack growth. The SCG performance in GB/T 13663 (new) is increased from 165h to 500h, which greatly improves the resistance of the material to slow crack growth. Ability to work.
5) Aging resistance. The adjustment is to evaluate the performance of raw materials and is no longer a performance requirement for pipes. The experimental items for pipe specimens after exposure have been adjusted, and the tensile strength of Kuqa hydraulic presses, which is both appropriate and flexible for performance evaluation, has been retained in the specification, but the original requirement for hydrostatic press strength of 80℃/165h has been changed to 80℃ /1000h new project. At the same time, the heat resistance is no longer measured, but is adjusted to consider the effects of different antioxidant systems – testing the welding performance of the electrofusion fittings of the pipes after exposure to the sun, in order to mainly determine the performance of the materials after climate and aging. Aging resistance is a measure of the ability of materials and products to resist the negative effects of sunlight, thereby determining the storage conditions and storage period of materials and products.
6) Improved the reduction requirements, that is, the test requirements for pipe specimens after reduction, and changed the original requirement of hydrostatic strength from 80°C/165h to 20°C/100h and 80°C/1000h. The reduction recovery performance is to test the performance of the pipe after it is flattened, repaired and repaired on site to ensure the performance of the pipe after repair. In the reduction reduction experiment, the interval L of the extruded pipe is calculated according to the column formula [6]:
L=2×k×ey,min where k——squeeze index (for dn≤250Mm, k=0.8; for 250Mm＜dn≤630Mm, k=0.9); ey,min——minimum wall thickness.
7) The oxidation induction time is changed from 200℃ to 210℃, which improves the oxidation resistance performance requirements.
8) The ash content requirements for raw materials/tubes and pipes have been increased respectively, raw materials ≤ 0.08% (molar mass), pipes/tubes ≤ 0.01% (molar mass), to ensure product quality before and after processing.
