PetroChina Dushanzi HDPE DMDN-8008 is a bimodal high-density polyethylene supplied as a black, carbon-black-stabilized pellet for solid-wall pressure pipe extrusion. The product is designated by the producer as a PE100 high-density polyethylene under ISO 12162, with a minimum required strength of 10.0 MPa at 20°C for 50 years when evaluated according to ISO 9080. This pressure rating places DMDN-8008 in the class of pipe materials used for municipal water distribution, buried natural gas conveyance, and industrial fluid transfer where long-term hydrostatic integrity is the controlling requirement. The grade is differentiated from general-purpose HDPE injection and blow molding resins by its lower melt flow rate, bimodal molecular weight distribution, and carbon black stabilization. Typical published values include a melt flow rate of 0.80 g/10 min at 190°C under 5.00 kg load and a density of 0.949 g/cm³. The bimodal molecular architecture combines a high-molecular-weight fraction that contributes to slow crack growth resistance with a lower-molecular-weight fraction that reduces viscosity during extrusion. The pellet is black due to carbon black loading in the range of 2.0–2.5% by weight, which provides ultraviolet stabilization for outdoor storage and buried service after pipe manufacture. DMDN-8008 is not intended for injection molding, thermoforming, or film conversion; its high melt viscosity and high molecular weight produce excessive filling pressure in injection molds and poor draw-down in film lines. The product is typically converted on grooved-feed single-screw extruders with vacuum calibration, and finished pipe is joined by butt fusion or electrofusion under model-specific procedures.

What Published Melt and Solid-State Properties Define DMDN-8008?

The nominal properties in the table are used for initial material qualification, while finished pipe must be tested for hydrostatic design compliance according to the relevant pipe standard. The melt flow rate of 0.80 g/10 min is significantly lower than injection molding HDPE grades, which commonly range from 4.0 g/10 min to 20.0 g/10 min. The low flow rate increases melt pressure during pipe extrusion but provides the high molecular weight needed for slow crack growth resistance. The density of 0.949 g/cm³ is higher than HDPE film grades and contributes to pipe ring stiffness while maintaining resistance to rapid crack propagation in pressurized service.

During pipe extrusion, DMDN-8008 is typically processed on grooved-feed single-screw extruders with L/D ratios between 30:1 and 33:1, or on counter-rotating twin-screw extruders when highly filled or wide-diameter pipe formulations are used. Barrel temperature profiles are set from 180°C in the feed zone to 200°C in the metering zone, with adapter and die-head temperatures maintained between 195°C and 215°C. The measured melt temperature at the adapter should remain in the range of 190°C to 215°C. Sustained operation above 230°C can induce oxidative chain scission, increase gel counts, and reduce long-term hydrostatic strength even if short-term tensile properties appear unchanged. Vacuum calibration is standard for solid-wall pipe; melt pressure at the breaker plate is controlled between 15 MPa and 25 MPa depending on die diameter, wall thickness, and line speed. Screw speed and line speed must be adjusted on the specific extruder to maintain the required melt pressure without exceeding the thermal degradation limit. Pre-drying is not normally required when pellets are stored in sealed containers and protected from condensation. If pellets are exposed to relative humidity above 60% or exhibit surface frost, hopper drying at 80°C for 2 hours is recommended to prevent surface splay and microvoids in thick-walled pipe. The material should not be processed above 230°C, and residence time in the die should be minimized to prevent thermal oxidation. Regrind of clean pipe scrap may be incorporated at low addition rates, but each converter must validate hydrostatic strength because the addition of regrind can alter slow crack growth resistance. Injection molding of DMDN-8008 is not recommended due to high melt viscosity; thin-wall injection tooling can experience short shots, weld-line brittleness, and clamp force requirements beyond the range of machines designed for general-purpose HDPE.

Slow Crack Growth Resistance and Hydrostatic Design Basis

Resistance to slow crack growth is the principal differentiator in pressure pipe service. DMDN-8008 uses a bimodal molecular weight distribution and controlled comonomer placement to resist crack initiation and propagation under sustained hoop stress. In the notched pipe test according to ISO 13479, pipe specimens are notched axially and tested under internal pressure at 80°C in water; failure time indicates slow crack growth resistance. PE100 formulations typically exceed 500 h in this test, and converter validation should confirm grade-specific failure times under the target diameter and wall thickness. The hydrostatic design basis is established under ISO 9080 by testing pipe samples at multiple temperatures and hoop stresses; the data are extrapolated to 50 years using the standard’s regression procedures. ISO 12162 then assigns the material a PE100 designation if the lower confidence limit of long-term hydrostatic strength satisfies 10.0 MPa at 20°C. This value is 25% higher than the 8.0 MPa minimum required strength for PE80 materials. As a result, a PE100 pipe can be designed with a thinner wall than a PE80 pipe for the same pressure rating, or it can carry a higher pressure at the same wall thickness, subject to the limits of ISO 4427 for water and ISO 4437 for gas. The carbon black content of 2.0–2.5% by weight also provides resistance to ultraviolet embrittlement, allowing outdoor storage and direct burial without additional UV stabilizer. However, carbon black cannot protect against oxidative attack from strong oxidizers, and chemical compatibility must be evaluated for each conveyed medium. Rapid crack propagation resistance is evaluated using the full-scale critical pressure test ISO 13477 at low temperature. PE100 compounds are designed to arrest rapid cracks; the material’s density and molecular weight are balanced to avoid brittle crack propagation at operating pressures.

When DMDN-8008 Is Benchmarked Against Unimodal HDPE Grades

In pressure pipe service, the distinction between DMDN-8008 and a general-purpose unimodal HDPE is defined by molecular architecture, melt flow, and long-term pressure rating. A unimodal injection HDPE with a comparable density of 0.949 g/cm³ but a melt flow rate of 4.0 g/10 min to 20.0 g/10 min can be molded into thin-wall articles, but it does not possess the high-molecular-weight fraction needed to meet PE100 hydrostatic requirements. The table below summarizes the key differences.

Compared with black PE80 pipe compounds, DMDN-8008 permits a higher allowable hydrostatic design stress but requires tighter extrusion temperature control. Compared with HDPE blow molding grades, DMDN-8008 has higher melt strength and a narrower processing window for parison formation; the same melt strength reduces sag in large-diameter pipe extrusion. In buried gas distribution, pipe produced from DMDN-8008 is joined by butt fusion under ISO 21307 or by electrofusion under ISO 12176; fusion parameters must be revalidated when changing from a unimodal pipe resin because the bimodal distribution changes melt flow and bead formation behavior.

In buried potable water networks, pipe manufactured from DMDN-8008 is installed as solid-wall PE100 pipe with diameters commonly ranging from 16 mm to 1200 mm, depending on the converter’s line capability and wall-thickness control system. Finished water pipe is specified under ISO 4427 and, in the Chinese market, GB/T 13663, while natural gas distribution pipe is specified under ISO 4437 and GB 15558.1. For potable water service, the pipe must also meet national hygiene requirements for contact with drinking water; the resin and pipe compound must be evaluated for migration and organoleptic properties according to the applicable national or regional certification scheme. In gas service, the same PE100 hydrostatic performance is combined with rapid crack propagation resistance evaluated under ISO 13477. Industrial applications include pressure transfer of slurries, dewatering lines, cooling water circuits, and chemical solutions where compatibility data indicate no reduction in long-term strength. DMDN-8008 is not suitable for continuous exposure to strong oxidizing acids, aromatic hydrocarbons, or chlorinated solvents without permeation and environmental stress cracking review. For chlorine-containing potable water systems, long-term performance should be assessed under the relevant chlorine resistance test method adopted by the certifying body, as chlorine accelerates surface embrittlement in polyethylene. Outdoor storage of black DMDN-8008 pipe is generally acceptable because the carbon black loading of 2.0–2.5% by weight limits ultraviolet degradation; however, prolonged storage in high-temperature conditions can accelerate antioxidant depletion. Before pressure testing, all butt-fusion and electrofusion joints must be cooled under manufacturer-specified pressure and alignment parameters to prevent residual stress concentrations in the pipe wall.