The Chevron Phillips Chemical HDPE 4600 (IPS) and HDPE 4700 grades are high-density polyethylene resins within the Marlex portfolio, supplied primarily for industrial piping, pressure pipe, sheet, and geomembrane extrusion. The 4600 designation is commonly associated with iron pipe size dimensioning in North American industrial and drainage piping, while the 4700 designation is specified where a lower melt-flow index and higher long-term hydrostatic strength are required. Both grades are ethylene-hexene copolymers with controlled short-chain branching and broad molecular weight distribution; the current ASTM D3350 cell classification should be verified against the latest PPI TR-4 listing. Manufacturer-published typical values place the 4600 nominal density at 0.946 g/cm³ and melt-flow index at 0.46 g/10 min (190 °C/2.16 kg); the 4700 grade shifts nominal density to 0.947 g/cm³ and melt-flow index to approximately 0.35 g/10 min.

How Do the 4600 and 4700 Grades Differ in Melt Rheology and Molecular Architecture?

The lower melt-flow index of 4700 corresponds to a higher weight-average molecular weight and greater apparent viscosity under constant shear. At 190 °C and 100 s⁻¹, 4700 exhibits elevated capillary viscosity relative to 4600, which increases head pressure and reduces output per screw revolution on a grooved-feed extruder. Both resins have broadened molecular weight distribution to balance shear thinning with processability, but 4700 shows a more pronounced strain-hardening response in extensional flow. This affects die swell, wall-thickness uniformity, and the onset of sharkskin during IPS pipe extrusion. For thin-wall pipe and small-diameter tubing, the melt fracture threshold shifts to lower output rates with 4700. Table 1 summarizes manufacturer-published typical values; current lot-specific datasheets govern final specifications.

On commercial single-screw extruders with 24:1 to 30:1 L/D ratio and grooved feed sections, 4600 is typically processed with barrel zone set points of 180–200 °C, adapter temperatures of 190–205 °C, and die temperatures of 195–210 °C. Melt temperature measured at the die entry is maintained between 190 °C and 210 °C to limit thermo-oxidative degradation. A barrier screw with a mixing section and a 20/40/60 mesh screen pack increases melt homogeneity and removes contamination. A gear pump after the screen changer reduces pressure pulsation and permits wall-thickness control within ±0.05 mm on pipe diameters up to 250 mm. The die land length-to-gap ratio is held at 15:1 to 25:1 to provide sufficient backpressure and reduce sharkskin. When 4700 replaces 4600, extruder amperage and die pressure increase by approximately 10–20 % at the same screw speed, and throughput may be reduced by 5–15 % unless melt temperatures are raised within the permissible upper limit. Unstable vacuum calibration, often caused by insufficient melt strength, appears as wall-thickness oscillation in pipe diameters above 160 mm; the higher melt strength of 4700 reduces this defect but requires higher vacuum pump capacity.

When Pipe Pressure Rating and Slow Crack Growth Interact

In pressure service, resin selection is governed by extrapolated hydrostatic strength evaluated according to ISO 9080:2022 and ASTM D2837-21. The 4700 grade is specified in PE4710-class pipe candidates because its higher molecular weight and controlled short-chain branching improve Stage II slow crack growth resistance under sustained internal pressure. The 4600 grade remains suitable for lower-pressure industrial and gravity drainage systems where long-term hoop stress is below the resin hydrostatic design basis. Slow crack growth is assessed by the Pennsylvania edge-notch tensile test, ASTM F1473, or the notched pipe test, ISO 13479; comparative data often show 4700 exceeding the PE4710 performance threshold at longer test times than 4600. Published data for this specific configuration is limited, and pipe-grade certification must be confirmed by the pipe manufacturer against final wall thickness and service temperature.

Butt fusion joining of 4600 and 4700 pipes is performed under ASTM F2620-22 with heater plate surface temperatures of 200–225 °C. Interfacial pressure during bead-up and fusion is set to 0.15 MPa ± 0.02 MPa for standard DIPS pipe. Electrofusion fittings for these resins are processed under ISO 21307:2017 using manufacturer-specific voltage and fusion time parameters. Because 4700 has higher melt viscosity, joint cooling time after electrofusion is extended by 10–20 % relative to 4600 to avoid brittle failure at the fitting interface. Injection molding of fittings from 4700 requires melt temperatures of 190–210 °C, mold temperatures of 10–40 °C, and injection velocities sufficient to prevent flow lines at the gate; published data for this specific configuration is limited.

In sheet and geomembrane extrusion, 4600 is processed on a single-screw extruder with a flexible lip die and a three-roll polishing stack. Roll temperatures are set at 80–100 °C for sheet thicknesses from 0.5 mm to 6 mm. Drawdown between die and roll stack is minimized to avoid orientation-induced anisotropy and to retain impact strength. For geomembrane, the resin is combined with 2.0–2.5 wt% carbon black and processed at die temperatures of 200–210 °C. The 4700 grade can be used for thicker geomembranes where high environmental stress crack resistance is required, but the higher melt viscosity may reduce line speed by 5–10 %.

Oxidative Induction Time, UV Stabilization, and Long-Term Hydrostatic Strength

Both grades are supplied with a hindered phenolic/phosphite antioxidant package; oxidative induction time at 200 °C by ASTM D3895-19 typically exceeds 20 min for unweathered pellets. For outdoor pipe and geomembrane applications, carbon black loadings of 2.0–2.5 wt% are required to meet ASTM D3350-21 weathering classification C or E. Natural grades without adequate carbon black or ultraviolet stabilizer have limited outdoor service life; prolonged ultraviolet exposure causes chain scission and can reduce elongation at break below the 50 % threshold within months. Hydrostatic testing at 20 °C and 60 °C under ISO 9080:2022 establishes the lower confidence limit of hydrostatic strength; selected pipe formulations based on 4700 commonly achieve a minimum required strength of 10 MPa at 20 °C for 50 years. The 4600 grade is more commonly assigned to non-pressure or low-pressure applications where long-term creep rupture is not the controlling design criterion.

Compared with the film extrusion grade Marlex HHM 5502, 4600 exhibits lower density and higher environmental stress crack resistance, which shifts application from thin film and blow molding to pipe and sheet. The injection molding grade Marlex HXM 50100 has a melt-flow index near 10 g/10 min, far above 4600 and 4700, making it suitable for thin-wall containers but unsuitable for pipe requiring creep rupture resistance. Within the same pipe family, 4600 provides lower head pressure and higher output than 4700, while 4700 provides lower melt-flow index and improved slow crack growth resistance. Selection between 4600 and 4700 depends on pipe pressure class, minimum required strength, and extrusion line capacity.

Determining Whether 4700 Improves Rapid Crack Propagation Resistance at Low Temperature

Rapid crack propagation in pressurized HDPE pipe is evaluated with the small-scale steady-state test, ISO 13477, or the full-scale test, ISO 13478. The 4700 grade is generally expected to show a lower critical pressure than 4600 at sub-zero temperatures because of its higher molecular weight and improved fracture toughness. In buried gas distribution pipe, the critical temperature for rapid crack propagation is often specified below -10 °C; pipe formulations containing 4700 may pass at wall thicknesses greater than those possible with 4600. However, rapid crack propagation resistance is also governed by pipe diameter, wall thickness, extrusion-induced orientation, and residual stress; resin-only comparisons are insufficient for final pipe qualification. Published data for this specific configuration is limited, and full-scale testing under ISO 13478 is required.

Compliance for potable water and industrial pipe applications is evaluated at the pipe or fitting level, not the pellet alone. Table 2 lists applicable standards that are commonly referenced in material certification packages.

At processing temperatures above 240 °C or hold-up times exceeding 10 min at melt temperatures above 210 °C, thermo-oxidative degradation reduces oxidative induction time and long-term hydrostatic strength. Surface condensation on cold regrind should be avoided because it produces splay and microvoids at die temperatures above 210 °C; pre-drying is not normally required for virgin pellets but may be necessary for regrind exposed to >60 % RH. These resins should not be combined with pro-degradant masterbatches or amine-based additives where long-term stabilization and potable-water certification are required.