LyondellBasell HDPE GF 4750 is a high-molecular-weight, bimodal high-density polyethylene supplied for extrusion blow moulding of large industrial containers, jerry cans, and technical parts. The grade is produced by the Hostalen Advanced Cascade Process, a cascade slurry polymerisation route that generates a controlled bimodal chain-length distribution. The low-molecular-weight fraction contributes to shear thinning and parison surface quality, while the high-molecular-weight fraction raises environmental stress crack resistance and melt strength. The nominal melt flow rate measured according to ISO 1133-1:2022 at 190 °C with a 21.6 kg load is 4.5 g/10 min; the density determined according to ISO 1183-1:2019 is 0.950 g/cm³. These values place the resin in the high-density polyethylene class, but test-specimen properties do not transfer directly to thick-walled, slowly cooled blow-moulded articles.

Table 1. Published nominal property values for LyondellBasell HDPE GF 4750.

The tensile yield stress of 26 MPa and flexural modulus of 1050 MPa place this resin in the intermediate-stiffness range for blow moulding HDPE. Lot-to-lot variation in these values can reach ±10 %, and the modulus is sensitive to cooling rate: slow cooling in a 5 mm container wall lowers crystallinity and can reduce flexural modulus relative to the ISO test plaque.

How Does Bimodal Chain Architecture Alter ESCR, Die Swell, and Yield Strength?

The bimodal chain-length distribution of GF 4750 separates the functions of molecular weight. In a unimodal high-density polyethylene, increasing molecular weight improves environmental stress crack resistance but also raises shear viscosity and complicates parison extrusion. The low-molecular-weight fraction in GF 4750 acts as an internal lubricant during melt processing, lowering shear viscosity at the die wall, while the high-molecular-weight tail forms tie molecules between crystalline lamellae. These tie molecules resist slow crack growth under stress. Environmental stress crack resistance is usually measured according to ASTM D1693-15, where notched specimens are immersed in 100 % Igepal CO-630 at 50 °C. Published F50 values for this specific grade exceed 300 h, though the result is strongly affected by notch quality, specimen thickness, and annealing. Conventional unimodal HDPE blow moulding resins of similar density often show F50 values in the 20–80 h range. The high-molecular-weight fraction also increases zero-shear viscosity and reduces parison sag on large accumulator-head tools. However, the material does not show the pronounced strain hardening of low-density polyethylene; wall-thickness programming therefore relies on die-gap adjustment and extrusion rate rather than on extension-dominated melt hardening. The yield stress is governed mainly by density and crystallinity, and at 0.950 g/cm³ the grade offers a stress-cracking resistance and stiffness balance that is difficult to obtain with unimodal resins without moving to lower density and losing modulus.

Table 2. Comparative property balance for GF 4750 and typical unimodal HDPE blow moulding grades.

The comparison in Table 2 is based on published ranges for conventional unimodal blow moulding HDPE; specific competitor grades may differ. The bimodal design shifts the environmental stress crack resistance–stiffness curve upward at a given density.

On accumulator-head blow moulding machines equipped with grooved feed sections and barrier screws in the 80–120 mm diameter range, the resin is typically processed with a flat-to-reverse barrel profile rather than a uniformly rising temperature profile. The feed zone is set at 170–190 °C, the compression zone at 190–210 °C, and the metering zone at 210–225 °C; head and die zones are held at 200–215 °C. Melt temperatures above 230 °C for extended residence times are not recommended because oxidative chain scission reduces the high-molecular-weight tail and degrades ESCR. Screw speed is adjusted to avoid excessive melt temperature; in a grooved-feed extruder, back pressure is controlled by downstream restriction rather than by a pressure-generating compression zone. Industrial data for this specific grade are limited, but transfer lines from 80 mm barrier screws on accumulator-head systems commonly show melt pressures between 25 MPa and 35 MPa before the accumulator. Die swell with this bimodal grade is moderate; tooling adjustments of 10–20 % on parison diameter are reported when converting from a lower-molecular-weight unimodal HDPE. Blow pressure is typically 0.5–0.8 MPa, and mould temperature is kept at 10–25 °C to maintain cooling efficiency without surface pinning. When wall thickness exceeds 3 mm, cooling time scales non-linearly with thickness, and raising mould temperature above 25 °C is generally not justified unless surface finish or residual stress reduction is specifically required. For very large shot volumes above 20 L, accumulator capacity, parison sag, and drop time must be validated on the specific machine because published data for high-volume tooling with this specific lot are limited.

Thermal degradation in bimodal high-molecular-weight HDPE is cumulative rather than instantaneous. Residence time distribution in accumulator-head machines is non-uniform; material in the hot core of the head can remain above 210 °C for 8–12 min during colour changes. Comparative melt flow measurements before and after purging indicate that a reduction of 0.2–0.5 g/10 min in 21.6 kg MFR may occur under prolonged hold time, with a corresponding loss in ESCR. Head and die zones should therefore be reduced to 190–200 °C during stoppages. Published data for this specific grade under production hold conditions is limited; the values cited derive from general bimodal HDPE processing experience.

In 20–60 L tight-head jerry can production, the parison programmer typically drops a thick-walled lower section to compensate for pinch-off thinning. The bimodal melt strength of GF 4750 allows a parison length-to-diameter ratio above 5:1 without severe sag. Mould pinch areas require adequate compression because high-molecular-weight tails can increase elastic recovery and affect weld integrity. Weld-line performance in drop tests according to ASTM D5276 or ISO 2248 is often evaluated before commercial use; published grade-specific drop-test data are limited.

When GF 4750 Should Be Evaluated Against Unimodal Medium-Molecular-Weight HDPE

The selection threshold for GF 4750 is usually the environmental stress crack resistance under ASTM D1693-15. When a container holds aggressive fluids such as surfactant solutions, agricultural adjuvants, or oxygenated fuels, and the required F50 exceeds 300 h, the grade is evaluated before a unimodal HDPE with similar density. The bimodal structure also provides higher parison stability on large tools with shot volumes above 20 L. However, the comparison is not unidirectional. On short shuttle presses, rotary blow moulders, or thin-wall containers below 1 mm, a medium-molecular-weight HDPE with a melt flow rate of 6.0–8.0 g/10 min at 21.6 kg may provide simpler parison control and shorter cycle times. GF 4750 is not typically recommended for injection moulding of thin-wall closures or for blow moulding of containers with flow-length-to-thickness ratios above 250:1, because the high zero-shear viscosity reduces tool filling and may accentuate weld-line thinning. Within the same LyondellBasell blow moulding portfolio, higher-flow bimodal HDPE grades are substituted when a lower melt viscosity is required for multi-cavity tools, whereas GF 4750 is retained for large single-cavity or dual-head production where ESCR and parison stability dominate. Published data for high-speed wheel machines running this specific resin are limited; therefore, drop-time and cooling-capacity trials should be conducted before tooling commitments.

Critical Operating Limits, Incompatibility Boundaries, and Resin Storage

For regulatory compliance, the final article rather than the pellet must be evaluated. Typical polyolefin food-contact frameworks include FDA 21 CFR 177.1520 and EU Regulation (EU) No 10/2011; migration testing is required under the actual contact time and temperature. For automotive fuel system components, permeation limits are set by the vehicle specification and may reference CARB/EPA test procedures; grade-specific emission data are not published. The material should not be exposed to strong oxidising acids, free halogens, or unsaturated hydrocarbons at elevated temperatures for extended periods. Pre-drying is not normally required for HDPE; however, if outdoor silo storage causes surface condensation, hopper drying at 60 °C for 2–4 h is used to remove visible surface water. Batch-to-batch variation in melt flow rate is controlled within certificate-of-analysis tolerances; converters should measure ISO 1133-1:2022 melt flow rate after adding colour concentrate because oxidative degradation from some carriers can reduce viscosity. Waste scrap from this grade can be reground and reintroduced at levels up to 20 % in many blow moulding operations, but repeated regrinding may shift ESCR and should be monitored through periodic ASTM D1693-15 testing.