PetroChina Dushanzi HDPE HD2840AA is a bimodal high-density polyethylene extrusion resin produced at the Dushanzi complex in Xinjiang. The grade is positioned for pressure pipe extrusion and is classified as PE 100 under ISO 12162:2009, with a minimum required strength of 10 MPa at 50 years and 20 °C. Mill certificates for the grade typically report a density of 0.959 g/cm³ and a melt flow rate of 0.30 g/10 min at 190 °C/5 kg. The 5 kg melt flow rate is used because the 2.16 kg value is too low for reproducible control in a high molecular weight pipe resin. The product is supplied as a natural extrusion grade; black pipe formulations require a separately dosed carbon black masterbatch, typically at 2.0–2.5 wt% for outdoor UV resistance.

The grade identifier follows the Dushanzi convention in which the HD prefix denotes high-density polyethylene, the four-digit sequence identifies the product number, and the AA suffix denotes the additive package and color form. Lot-to-lot control is maintained on melt flow rate, density, tensile yield stress, and oxidative induction time; the certificate of analysis for each batch should be checked before extrusion.

Why does the bimodal molecular weight distribution control the ductile-to-brittle transition?

The bimodal distribution separates the resin into a low molecular weight fraction that reduces melt viscosity at high shear and a high molecular weight fraction that builds chain entanglements and tie molecules in the amorphous regions. The comonomer is preferentially incorporated into the high molecular weight chains, which raises the areal tie-molecule density and shifts the ductile-to-brittle transition to longer times under a given hoop stress. This transition is not captured by short-term tensile tests; it is evaluated by the notched pipe test ISO 13479:2009 and by the notched constant tensile load test ASTM F1473. In long-term hydrostatic strength testing to ISO 9080:2012, the lower predictive limit at 50 years/20 °C remains above 10 MPa, which supports PE 100 classification. The higher minimum required strength permits an SDR 11 pipe to carry PN 16 when the design coefficient C = 1.25 is applied. The same design stress permits an SDR 17 pipe to carry PN 10.

Slow crack growth in polyethylene pipe is characterized by the formation of fibrillar bridges behind a propagating crack tip; the failure time scales with the density of tie molecules and the concentration of high molecular weight chains. In a bimodal resin, the high molecular weight fraction is copolymerized to suppress lamellar thickness uniformity and to increase interlamellar entanglement. This produces a discontinuous crack propagation path and a longer stable crack growth stage. The notched pipe test according to ISO 13479:2009 at 80 °C and 4.6 MPa hoop stress is used to screen batches; survival beyond 500 h without brittle failure is typical for PE 100 grades of this type. The resistance to rapid crack propagation, measured by the S4 test ISO 13477, is also relevant for gas and cold-water pipe; the bimodal high molecular weight structure lowers the critical pressure and critical temperature relative to unimodal resins. Published data for this specific configuration is limited, and pipe manufacturers must run the ISO 13477 test on the finished pipe formulation.

Typical property profile and certification matrix

Published values for HD2840AA are summarized in Table 1. These values are class-typical for Dushanzi PE 100 pipe resin; the certificate of analysis for each lot carries the controlling test results.

For potable water service in China, the finished pipe is evaluated separately under GB/T 17219 and GB/T 13663.2-2018. The resin alone does not carry potable-water approval; the pipe manufacturer must validate the complete formulation, including any color masterbatch, processing stabilizers, and regrind ratio. The use of regrind from clean, unpigmented HD2840AA pipe is generally limited to 10–15 wt% for pressure pipe applications, but the permitted level is determined by the manufacturer's internal hydrostatic qualification rather than by the resin supplier. Gas distribution applications are not implied by the PE 100 classification alone; pipe for gaseous fuels must be qualified to ISO 4437 or the applicable national gas standard with the specific black compound, not with the natural resin.

Temperature excursions above 240 °C shorten oxidative induction time on grooved-feed extruders

HD2840AA processes on single-screw extruders with grooved feed zones and barrier screws at L/D ratios from 30:1 to 36:1. For a 75 mm grooved-feed extruder running SDR 11 pipe, barrel temperatures are usually set from 190 °C to 220 °C and the melt temperature is maintained between 210 °C and 225 °C. A melt temperature above 240 °C initiates oxidative chain scission that reduces the oxidative induction time below 20 min and increases the risk of surface sharkskin and pipe wall inhomogeneity. Extruder head pressure typically ranges from 20 MPa to 40 MPa depending on die land length and output; excessive backpressure above 50 MPa on a 75 mm line can shear-heat the melt and shorten screen pack life.

Screw design should include a grooved feed bushing with a water-cooled hopper throat, a barrier flight section for dispersion of the high molecular weight fraction, and a mixing head with moderate shear. A gear pump may be installed between the extruder and die to stabilize output and reduce melt temperature variation. The die land ratio and spider leg geometry influence weld-line strength; for thick-wall SDR 11 pipe, a spiral mandrel die with optimized flow channels prevents melt stagnation. Pre-drying is not required for HD2840AA under normal storage conditions because polyethylene has negligible equilibrium moisture absorption. If cold resin is transferred into a warm processing hall with relative humidity above 80 %, condensation on pellet surfaces can cause melt pinholes and die lip buildup. In such cases, hopper drying at 70–80 °C for 2–4 h is applied. The resin should not be blended with amine-based additives, as these can interact with the hindered phenolic stabilizer package and reduce long-term thermal stability.

When HD2840AA is substituted for a unimodal PE80 grade on an existing pipe line

The substitution is not a drop-in change because the melt rheology differs. The bimodal resin shows higher viscosity at low shear and stronger shear thinning than a unimodal PE80 grade. Operators typically observe a higher head pressure at the same screw speed, and the melt temperature may rise by 5–10 °C if screw speed is not reduced. The recommended sequence is to retain the same barrel profile but reduce screw speed by 10–15 % until the melt temperature returns to the 210–225 °C window. Pipe wall thickness can be reduced when moving from PE80 to PE100 at the same nominal pressure, because the higher minimum required strength permits a higher SDR. For PN 10 at 20 °C, PE100 can use SDR 17 instead of SDR 13.6, which reduces material consumption per metre and narrows the die gap if the same outside diameter is retained.

On the same production line, the switch to HD2840AA changes the calibration and haul-off settings because the higher melt elasticity can alter die swell and tubular wall thickness distribution. Extruder head pressure rises with the high molecular weight fraction, and the die gap may need adjustment to maintain the specified wall eccentricity. Vacuum calibration tanks with chilled water at 15–25 °C are used for dimension control; a water temperature below 10 °C can chill the pipe surface and increase residual stress. The final pipe must pass the hydrostatic pressure test and the melt mass flow rate and density checks specified in the applicable product standard. The resin is not intended for rotational molding, injection molding of thin-wall packaging, or film extrusion; the high molecular weight fraction produces excessive melt pressure and poor drawdown in those processes.

In comparison with high-density polyethylene blow molding grades from the same production site, HD2840AA shows a lower 5 kg MFR and a higher notched pipe test survival time. Blow molding grades typically require high melt strength and controlled sag, whereas HD2840AA is formulated to maximize slow crack growth resistance under long-term internal pressure. The difference in molecular architecture means that HD2840AA should not be used for extrusion blow molding of bottles or containers; the high molecular weight fraction leads to high head pressure and low parison extensibility. Conversely, blow molding HDPE should not be substituted into pressure pipe because the lower tie-molecule density shortens the hydrostatic life and can cause brittle failure after a few years under hoop stress. Users should not dilute HD2840AA with lower-molecular-weight HDPE to improve processability, because the slow crack growth resistance and the PE 100 classification depend on the bimodal distribution and the high molecular weight fraction. Dilution with 10 wt% of a high-flow HDPE can reduce the lower predictive limit below 10 MPa and invalidate the hydrostatic design basis.