Borealis HE3498-LS is a bimodal high-density polyethylene pressure-pipe compound supplied as a ready-to-extrude black compound. The grade is classified as PE100 under ISO 12162:2009, which means that long-term hydrostatic strength data extrapolated by ISO 9080:2012 produce a lower predictive limit σ_LPL of 10 MPa at 20 °C and 50 years. The compound is intended primarily for extrusion of pressure pipe for water transportation, industrial fluid service, and buried drainage under pressure. The typical density of HE3498-LS is 959 kg/m³ when measured according to ISO 1183-1:2019. The melt mass-flow rate at 190 °C and 5 kg is published as 0.85 g/10 min by ISO 1133-1:2022. The carbon black content is controlled in the range 2.0–2.5 wt% under ISO 6964:2019, which provides ultraviolet stabilization in black pipe. Oxidation induction time at 200 °C is reported as greater than 20 min under ISO 11357-6:2018. These values place HE3498-LS among the higher-viscosity PE100 extrusion grades; the low-sag formulation distinguishes it from general-purpose PE100 pipe compounds in thick-wall and large-diameter pipe production.

Extrusion of HE3498-LS is normally performed on grooved-barrel single-screw extruders with screw lengths in the range 30:1 to 36:1 L/D. The grooved feed section stabilizes solids conveying against the high viscosity of the melt. Barrel temperatures from feed to die are typically set between 190 °C and 220 °C, with die-head temperature held near 210 °C to balance pressure drop and melt strength. Melt temperatures above 240 °C are not recommended because the thermo-oxidative stabilizer package is consumed faster and the oxidation induction time can fall below the level required for long-term pressure service. Polyethylene is not hygroscopic, so pre-drying is generally unnecessary; however, surface condensation from storage in cold conditions should be removed before the material enters the extruder throat. Pipe production of black HDPE grades does not require color masterbatch addition, because the carbon black and stabilizer package are already compounded into the pellet.

Why Does the Low-Sag Designation Alter Wall-Thickness Control in Thick-Wall Pipe?

The LS designation in Borealis HDPE pipe grades refers to low-sag extrusion behaviour. In large-diameter or thick-wall pipe production, the molten extrudate exiting the die is unsupported between the die face and the calibration sleeve. Gravity-induced flow in this interval can produce wall-thickness eccentricity and reduce the hydraulic pressure rating of the finished pipe. The phenomenon becomes severe when the standard dimension ratio SDR is low because the molten wall is thick and retains heat longer. HE3498-LS is formulated to increase melt strength and zero-shear viscosity relative to a standard PE100 grade with the same hydrostatic classification. The higher melt strength reduces the initial sag rate without requiring the melt temperature to be lowered below the processing window. Standard PE100 grades without low-sag control may achieve the same wall-thickness tolerance only by reducing melt temperature, which raises die-head pressure and may cause melt fracture, or by reducing line speed, which lowers output. Published pipe production data for this specific configuration is limited, but the formulation difference is most relevant for low-SDR, thick-wall pipe where the free-span interval cannot be shortened. Processors comparing HE3498-LS with standard PE100 grades should verify that the extruder drive has sufficient torque capacity, because the higher low-shear viscosity of the LS grade increases energy demand at the screw even when the die pressure reading remains similar.

Hydrostatic Strength Classification and Design-Stress Basis for PE100 Pipe Systems

HE3498-LS derives its pressure-pipe suitability from the PE100 long-term hydrostatic strength classification. Under ISO 9080:2012, pipe specimens are tested at multiple temperatures and hoop stresses, and the data are extrapolated to 50 years at 20 °C. The lower confidence limit σ_LPL for PE100 is 10 MPa. Under ISO 12162:2009, the minimum required strength MRS is 10 MPa. For water supply pipe, ISO 4427-2:2019 applies a service design coefficient C of 1.25; the design stress σ_s is therefore 8.0 MPa. The pressure rating of a pipe section is calculated from the equation PN = 2 σ_s / (SDR − 1), where SDR is the outside diameter divided by wall thickness. For an SDR 11 pipe, this gives PN = 2 × 8.0 MPa / 10 = 1.6 MPa, commonly designated PN 16. For an SDR 17 pipe, the result is 1.0 MPa, designated PN 10. Because HE3498-LS is a PE100 material, it permits either higher pressure rating at the same wall thickness compared with PE80 materials, or reduced wall thickness at the same pressure rating. The calculation is valid only when the pipe is manufactured and tested according to the complete ISO 4427 system, including pipe-level hydrostatic strength tests and dimensional checks.

Compared with PE80 pipe-grade materials, HE3498-LS uses the 10 MPa MRS basis. A PE80 material is classified with MRS of 8 MPa; under the same 1.25 design coefficient, the design stress is 6.4 MPa. For a given pressure rating, a PE100 pipe may therefore be designed with a higher SDR and lower wall thickness than a PE80 pipe, although the PE80 material may process with lower extruder head pressure. Within the PE100 class, general-purpose grades are usually formulated for high line speed in small-diameter pipe, whereas HE3498-LS is formulated for thick-wall dimensional control and slow crack growth resistance. The difference is not primarily density; black HDPE pressure pipe compounds typically remain in the 950–960 kg/m³ range. The determinant factor is the molecular architecture and stabilizer formulation.

In drinking-water applications, pipe-grade HDPE must also satisfy organoleptic and migration requirements set by national approvals. The black carbon black dispersion of HE3498-LS should be assessed according to ISO 6964:2019 and ISO 18553. Poor dispersion can produce local defects that act as stress concentrators in pressure pipe. The LS formulation does not remove the need for dispersion testing; it only addresses melt strength and slow crack growth behaviour.

Pipe production with HE3498-LS on a grooved-barrel single-screw extruder uses conventional pipe-grade screw geometry with a mixing section to disperse the carbon black and stabilizer package. The screw should be capable of stable operation against high head pressure at the melt temperature of 190 °C to 220 °C. The die is commonly a spiral mandrel design; spiral angle and land length are selected to avoid weld lines and control wall-thickness distribution. Vacuum calibration is used to set outside diameter during cooling. When the melt temperature is reduced below 180 °C, melt fracture can appear at the pipe surface, which establishes the low-temperature boundary of the production window.

Extrusion lines that handle HE3498-LS often include an upstream gravimetric feeding system to control pellet feed stability; feed interruption can produce pressure oscillation and wall-thickness variation. The gravimetric feeder should be calibrated for black HDPE pellets with the bulk density specified by the supplier. At high screw speed, the grooved barrel is cooled to prevent premature melting in the feed zone, which would reduce conveying efficiency. The melt pump, if installed, must be rated for the head pressure and melt viscosity of the LS grade; melt pumps should not be used to compensate for poor screw design because excessive shear heating can exceed the 240 °C limit.

Processing Window Boundaries for Thick-Wall Pipe Extrusion with HE3498-LS

Dimensional stability in thick-wall pipe extrusion is controlled by the interaction of melt temperature, die geometry, and cooling rate. The practical processing window for HE3498-LS is bounded at the upper end by thermal degradation and at the lower end by melt fracture. At melt temperatures above 240 °C, the stabilizer package is consumed more rapidly, and the oxidation induction time measured by ISO 11357-6:2018 decreases; long residence time above this temperature should therefore be avoided. At melt temperatures below 180 °C, the viscosity increases to a range where die-head pressure rises and surface melt fracture may occur. The precise low-temperature limit depends on screw speed, die gap, and molecular weight distribution. In grooved-barrel extruders with L/D ratios in the range 30:1 to 36:1, the specific mechanical energy input is influenced by the high zero-shear viscosity of the LS grade. Higher torque is reported relative to general-purpose PE100 grades at comparable screw speed and throughput. A screw with sufficient depth in the feed zone and a low-compression mixing profile may be required to keep drive current below the extruder motor rating.

The die must not be operated above the recommended melt temperature to compensate for an undersized drive, because this increases sag and erodes the dimensional benefit. In contrast, lowering the die temperature to reduce sag can raise head pressure and create surface defects. This conflict between sag and melt fracture defines the production window. Equipment manufacturers’ technical bulletins should be used to establish the initial screw and die configuration; the final settings are validated on the actual pipe line because extruder backpressure depends on the specific die geometry and cooling length. Regrind addition should be validated at the pipe level because hydrostatic strength classification is a property of the finished pipe, not the pellet alone.

When Installation Conditions Make Slow Crack Growth the Dominant Failure Criterion

Pressure pipes installed in rocky or uneven soil without full sand bedding are exposed to local point loads from stones and other hard inclusions. Under long-term internal pressure, these point loads can create stress concentrations that initiate slow crack growth before the material would fail by general creep rupture. For such service conditions, the relevant material test is not only the hydrostatic strength classification but also the resistance to slow crack growth. The notched pipe test according to ISO 13479 is used to evaluate this mechanism. The test introduces a defined external notch on the pipe and applies internal pressure at elevated temperature; the time to brittle failure is recorded. A typical acceptance criterion for PE100 pressure pipes is a minimum failure time in the order of 500 h under the specified hoop stress and test temperature, but the exact requirement depends on pipe dimension and project specification. HE3498-LS is designed with a higher slow crack growth resistance than general-purpose PE100 pipe grades because its bimodal molecular architecture increases the high-molecular-weight fraction and tie-molecule density. Published data for HE3498-LS in this specific notch configuration is limited, so pipe-level test reports under ISO 13479 should be requested when the installation specification explicitly demands slow crack growth qualification.

In a pipe production campaign targeting SDR 11 PN 16 water pipe, HE3498-LS is typically evaluated when the pipe outside diameter and cooling length are such that the extrudate cannot be supported immediately after the die. Under these conditions, the low-sag melt behaviour reduces wall-thickness eccentricity during the free-span interval. The pipe manufacturer must still confirm that the extruder drive can sustain the higher torque at the desired line speed and that the melt temperature remains below 240 °C. Final qualification of the pipe is performed according to ISO 4427-2:2019; the resin-level PE100 classification under ISO 12162:2009 does not replace pipe-level hydrostatic pressure testing.