Mitsui Chemicals HDPE 6900 is a high-density polyethylene grade supplied for extrusion blow moulding of rigid technical parts, industrial containers, and automotive fluid reservoirs. Representative manufacturer-published values include a nominal density of 0.956 g/cm³ determined according to ISO 1183-1 and a melt flow rate of 0.30 g/10 min at 190 °C under a 2.16 kg load determined according to ISO 1133-1:2022. The low melt flow rate reflects a high weight-average molecular weight, which contributes to elevated parison melt strength, high environmental stress-crack resistance, and slower extruder throughput relative to lower-viscosity HDPE grades. The grade differs from conventional unimodal blow-moulding resins through a broader molecular weight distribution and a controlled comonomer short-chain branching distribution, which shifts the balance among stiffness, ESCR, and die swell. The data below define the material and processing boundaries that govern where HDPE 6900 can replace lower-density or lower-molecular-weight HDPE grades and where substitution is technically inappropriate.

Which melt-flow and density values define HDPE 6900?

At a molecular level, a density of 0.956 g/cm³ corresponds to a crystalline fraction of approximately 68% when measured by density gradient column per ISO 1183-1, assuming a fully amorphous HDPE density of 0.855 g/cm³ and a crystalline cell density of 1.000 g/cm³. The melt flow rate of 0.30 g/10 min places this resin in the high-viscosity blow-moulding class. Dynamic oscillatory rheometry at 190 °C under nitrogen generally yields a zero-shear viscosity between 5.0 × 10⁴ Pa·s and 1.5 × 10⁵ Pa·s for comparable high-molecular-weight HDPE grades. The shear-thinning index over a shear-rate range of 10 s⁻¹ to 100 s⁻¹ is approximately 0.28, which means that viscosity drops significantly inside die lands and permits parison extrusion at tolerable head pressures.

The broad molecular weight distribution carries a high-molecular-weight tail that increases the density of tie molecules at lamellar boundaries. This architecture raises slow crack growth resistance as measured by ISO 16770 full-notch creep testing at 50 °C in a 2% nonylphenoxy poly(ethyleneoxy) ethanol solution. The tie-molecule concentration is the primary microstructural parameter controlling failure time under constant load. The high-molecular-weight fraction also raises extensional viscosity at Hencky strain rates of 0.1 s⁻¹ to 1.0 s⁻¹, producing strain-hardening coefficients of 2.0 to 3.5 in Sentmanat extensional rheometer measurements.

These values are not specification release limits; they are supplied as typical values from current manufacturer technical documentation and must be verified against the certificate of analysis for the lot in use. The relatively high density places HDPE 6900 among stiff HDPE blow-moulding grades, which is relevant for stack-load and top-load requirements in 25 L to 220 L containers.

Comparative Position Against Unimodal HDPE Blow-Moulding Grades

Compared with unimodal HDPE resins of similar nominal density and melt flow rate, HDPE 6900 shifts the failure envelope from brittle slow crack growth toward ductile yielding. Under ASTM D1693 condition B with 10% Igepal CO-630 at 50 °C, broad-MWD blow-moulding grades of this type have F50 times above 300 h, while unimodal grades of similar density may fall below 100 h. The actual value for HDPE 6900 on a compression-moulded notch strip depends on cooling rate and residual thermal stress. Moulded containers with wall thickness below 1.5 mm can show shorter ESCR because rapid cooling reduces the time available for tie-molecule formation.

The notched impact strength at 23 °C is 18 kJ/m² by ISO 179-1/1eA, which is higher than many 0.955 g/cm³ unimodal grades reporting 8–12 kJ/m². This difference is relevant for drop resistance of large containers. The trade-off is throughput: at identical screw speed on a 60 mm smooth-bore extruder with 25:1 L/D, a 0.30 g/10 min high-molecular-weight HDPE typically produces 10–20% lower output than a 0.60 g/10 min unimodal HDPE. Die swell also differs. HDPE 6900 produces a moderate annular die swell, commonly 35–45% at a shear rate of 50 s⁻¹, compared with 25–35% for lower-viscosity injection-moulding HDPE grades. Die tooling must therefore be adjusted when substituting from a 1.0 g/10 min grade; the die gap is often enlarged by 0.3–0.6 mm to maintain the same final container wall thickness.

Substitution from an injection-moulding HDPE grade with a melt flow rate above 10 g/10 min is technically inappropriate for thin-wall caps or crates because the filling pressure of HDPE 6900 would exceed 120 MPa on parts with flow-length-to-wall-thickness ratios above 200:1. Replacing a low-density polyethylene grade with HDPE 6900 in a rigid container increases top-load strength and chemical resistance but changes the ESCR response under certain detergent formulations; selection must therefore be based on chemical compatibility with the intended filling product.

When shuttle-type extrusion blow moulders are operated at high back pressure

On a shuttle-type extrusion blow moulder with an 80 mm grooved-barrel extruder having an L/D of 24:1, barrel zone set points are typically 150 °C, 170 °C, 190 °C, and 210 °C, with the head and die set at 205 °C. Melt temperature at the die entry is held between 190 °C and 220 °C. Operation above 230 °C for more than 45 s causes thermo-oxidative chain scission, increasing MFR and lowering parison melt strength; the resulting containers show higher wall-thickness variation and reduced top load. If die-head pressure rises above 30 MPa because of cold start-up or a partially blocked screen pack, sharkskin melt fracture appears on the outer parison surface at shear rates above 500 s⁻¹. The corrective sequence is to raise die temperature to 205 °C, reduce screw speed by 5–10 rpm, and purge with a low-viscosity HDPE grade for 5–10 min.

Blow pressure is maintained at 0.6–0.9 MPa, with a blow time of 12–18 s and mould temperature of 15–30 °C. For a 20 L jerrycan with a projected parting-line area of 0.08 m², a clamp force of 130–180 kN is required to prevent mould flash. Vertical wall-thickness variation is kept below 0.4 mm by programming the die gap between 1.4 mm and 2.4 mm; parison sag is controlled by keeping drop time below 2.5 s. On an 80 mm grooved-barrel extruder, head pressure at 40 rpm is typically 18–25 MPa, and extruder motor load is 8–12% higher than that for a 0.6 g/10 min HDPE.

Accumulator-head machines processing 60 L to 220 L open-top drums require different die-gap management because the parison length can exceed 1.5 m. The high-molecular-weight fraction reduces gravitational sag, but a drop time below 2.0 s on a 120 mm 25:1 L/D accumulator machine can create uneven wall thinning near the top chime. Pinch-off weld-line integrity is maintained by holding pinch-off land temperature at 20–25 °C and blow pressure at 0.8–1.0 MPa. Under these conditions, filled drums in drop impacts generally fail by sidewall ductile tearing rather than pinch-off splitting. The power draw on 120 mm accumulator extruders running HDPE 6900 is 10–15% higher than that for a 0.6 g/10 min HDPE at the same screw speed because of the higher melt viscosity and shearing work.

In slow crack growth testing according to ISO 16770 at 50 °C under a notched constant tensile load, the failure time of HDPE 6900 depends strongly on cooling rate from the melt. Quenching at an average rate above 30 °C/min reduces tie-molecule formation and can lower the failure time by 20–30% relative to slow cooling at 5 °C/min. Moulders of thick-walled chemical containers therefore avoid mould temperatures below 10 °C when maximum ESCR is required. The same mechanism explains why weld lines and pinch-off zones are the critical sites for environmental stress cracking; these regions experience high cooling rates and molecular orientation perpendicular to the weld plane.

Regrinding 6900-Based Containers Without Compromising ESCR

Regrinding HDPE 6900 containers is feasible when the regrind stream is free of polypropylene caps, PET labels, metal foils, and paper residues. In closed-loop trials on 25 L containers, regrind addition up to 25 wt% produced no measurable reduction in ESCR when the regrind was dried at 80 °C for 2 h and processed at a melt temperature of 205 °C. At regrind levels above 40 wt%, the compounded stream shifted from 0.30 g/10 min to 0.44 g/10 min after three reuse cycles, and notched Charpy impact strength decreased by approximately 12%. These changes reflect chain-scission damage accumulated during repeated extrusion.

Pre-drying is required when regrind has been stored at relative humidity above 60%; surface moisture above 0.05 wt% creates parison pockmarks and weld-line porosity. Processing with high regrind content above 220 °C accelerates molecular weight loss and increases odour from oxidized low-molecular-weight fractions. Closed-loop regrind use should therefore be limited to 25–30 wt% in food-contact and hazardous-materials containers unless lot-specific ESCR and organoleptic results support higher addition.

For food-contact packaging, HDPE 6900 can be evaluated under the following standards. End-use suitability must be established by migration testing on the finished article because the resin alone does not guarantee compliance with all food simulant requirements.

The RoHS directive does not normally apply to unfilled HDPE because heavy metals and brominated flame retardants are not intentionally added; however, downstream converters must verify that masterbatch colourants and processing aids do not introduce restricted substances above the maximum concentration values in Directive 2011/65/EU Annex II. For REACH, a supplier declaration of compliance is required for each lot; no SVHC above 0.1% w/w should be present in the resin or added preparation.

The grade is not designed for injection moulding of thin-wall caps or for blown film. In injection moulding, the high melt viscosity raises required filling pressures above 120 MPa on parts with flow-length-to-wall-thickness ratios above 200:1. In blown film, the low melt index restricts bubble expansion and can create an unstable melt curtain. These limitations distinguish HDPE 6900 from general-purpose HDPE grades with a melt flow rate of 0.7–1.5 g/10 min and from film grades with a high-load melt index above 4 g/10 min at 190 °C/21.6 kg.