Borealis HDPE HE2591 is a high-density polyethylene grade developed for extrusion blow moulding of rigid containers in the 0.5 L to 10 L capacity range. The material is supplied as pelletised resin and carries a nominal density of 958 kg/m³ when tested according to ISO 1183-1, and a nominal melt flow rate of 0.30 g/10 min at 190 °C under a 2.16 kg load using ISO 1133-1. These values indicate a medium-high molecular weight HDPE with a balance between melt strength and processability. The molecular structure includes a high-molecular-mass fraction that raises parison stability and improves environmental stress-crack resistance compared with general-purpose blow moulding grades at the same density. In production practice, the grade is processed on single-station and double-station shuttle blow moulding machines, continuous shuttle lines, and accumulator-head machines. The melt temperature is normally kept between 180 °C and 220 °C. Operation above 230 °C leads to parison sag, unacceptable bottle weight variation, and weaker pinch-off welds.

Typical Property Values and Specification Basis

The values in Table 1 are typical lot-average data published by the manufacturer and do not constitute minimum specification limits. Melt flow rate and density are the primary lot-release parameters; mechanical and thermal data are supplied as indicative values. Tensile properties are measured on compression-moulded plaques at 23 °C and 50 % relative humidity according to ISO 527-2/1B. Vicat softening temperature is determined according to ISO 306/A50 with a 50 N load and a 50 °C/h heating rate.

The tensile modulus near 1100 MPa provides buckling resistance for stacked household chemical bottles. The Vicat softening temperature near 127 °C is relevant for short-duration warm-fill or warm-warehouse exposure but does not establish continuous service temperature. The grade is not designed for contents held above 60 °C under constant internal pressure. At higher temperatures, creep in the neck and shoulder regions can reduce closure back-off torque, especially with low-clearance polypropylene closures or induction-seal liners that impose residual stress on the finish.

For extrusion blow moulding on a 60 mm grooved-barrel extruder with 24:1 L/D and a continuous shuttle line, the temperature profile is normally set from 180 °C at the feed section to 210 °C at the die head. The die zone is maintained between 200 °C and 215 °C to avoid melt fracture while limiting parison sag. Blow mould temperature is maintained between 10 °C and 30 °C. Mould temperatures above 40 °C extend cycle time and can increase post-mould shrinkage above 2 % in the hoop direction. For a 1 L bottle with 0.8 mm nominal wall thickness, cooling time is commonly 8 s to 12 s at a mould temperature of 15 °C. The grade accepts up to 25 wt% clean, unpigmented regrind from the same bottle production without substantial ESCR loss. External regrind, mixed-colour post-consumer reclaim, or closure resin contamination is not recommended because low-viscosity fractions alter die swell and pinch-off integrity. If pellets are stored at relative humidity above 60 %, surface condensation can cause splay in thin wall sections; pre-drying at 80 °C for 2 h is sufficient.

Pinch-off weld strength in the tail flash and handle regions depends on melt temperature and mould closing speed. A melt temperature below 190 °C can produce cold pinch-off welds with reduced low-temperature drop impact resistance at 0 °C. Mould closing speed should be set so that the pinch-off zone is compressed before skin solidification. On a 2 L handleware bottle, pinch-off thickness after deflashing is typically 0.4 mm to 0.6 mm. Inadequate pinch-off compression leads to base-seam cracking during drop impact testing according to ASTM D2463. A parison programmer with 20 points per cycle reduces wall-thickness variation and improves top-load strength. On accumulator-head machines, parison drop time should be minimised because the high-molecular-mass fraction can cause parison curl if the die gap is uneven.

Table 2 summarises a conservative processing window for shuttle-type blow moulding equipment. These ranges are starting values and must be adjusted to the specific clamp force, parison programmer, and cooling system.

Rheological measurements on the grade show a pseudoplastic melt with strong shear-thinning at low shear rate. At 190 °C and a shear rate of 100 s⁻¹, the apparent melt viscosity is approximately 3,000 Pa·s to 4,500 Pa·s; published data for this specific shear rate is limited. The high viscosity increases die-head pressure on accumulator machines and may require die-gap adjustment when switching from a lower-viscosity grade. The addition of 2 wt% colour concentrate can alter shear viscosity and die swell; concentrates based on low-molecular-weight carrier resins may increase die swell by up to 10 % and require parison programmer adjustments. Thermal degradation follows oxidative chain scission rather than crosslinking at blow moulding temperatures. At melt temperatures above 240 °C, the melt flow rate increases and ESCR decreases before visible yellowing occurs. Die-exit melt temperature should be confirmed with an insertion thermocouple, not inferred from barrel set points. A die-exit melt temperature above 230 °C indicates excessive shear heating or residence time and requires reduction of screw speed or barrel temperature.

What separates HE2591 from general-purpose 0.958-density HDPE in aggressive detergent formulations?

General-purpose HDPE grades at 0.958 g/cm³ often fail by environmental stress cracking when exposed to surfactants, bleach, or fragrance esters under residual stress. HE2591 differs because the polymerisation process produces a broader molecular weight distribution and a higher concentration of tie molecules connecting crystalline lamellae. This increases the energy required for craze initiation and slows crack propagation in the amorphous phase. Under ASTM D1693 condition B with 100 % Igepal CO-630 at 50 °C, the manufacturer’s typical F50 value for HE2591 is reported above 300 h, while many general-purpose 0.958 g/cm³ blow moulding grades fall below 100 h. Under ISO 22088-3 bent-strip testing, the ranking is similar. The consequence in package design is that HE2591 can often be used at a lower wall thickness for aggressive household chemical formulations without an increase in service cracking. However, bottle geometry, neck finish design, closure insertion force, and filling-line shock loads also control ESCR performance, so a full bottle test with the actual chemical formulation is required.

The ESCR advantage is not detectable from melt flow rate or density alone. A standard lot-check on density and MFR cannot distinguish HE2591 from a lower-ESCR grade in the same density band because the difference resides in the chain architecture and short-chain branching distribution. The high-molecular-mass chains carry load across the amorphous regions; if these chains are selectively removed by thermal or oxidative degradation during processing, ESCR drops even when density and MFR remain within specification. For this reason, melt residence time above 230 °C or repeated recycling through the extruder should be avoided. Processing with high-shear screws at screw speeds above 60 min⁻¹ can generate shear heating of 5 °C to 10 °C above the barrel set point and mechanically degrade the high-molecular-mass fraction. On accumulator-head machines, residence time in the accumulators should be kept below 20 min at 210 °C to avoid oxidation-induced ESCR loss.

Low-temperature performance is evaluated by filled-bottle drop impact according to ASTM D2463. A 1 L bottle with an average wall thickness of 0.8 mm typically withstands a 1.2 m drop at 0 °C when filled with water, but results with surfactant-containing formulations are lower because the formulation plasticises the surface and lowers craze initiation energy. Top-load strength should be measured on empty bottles according to ISO 12048, with a compression rate of 25 mm/min. The measured top-load yield depends more on bottle shape, wall distribution, and pinch-off quality than on resin selection alone.

For incoming inspection, a single-point ESCR test under ASTM D1693 condition B at 50 °C is useful but has high variability. A more reliable ranking is obtained with a minimum of 5 specimens per lot and reporting the F50 value. If ESCR testing is not available at the customer site, a notched constant tensile load test under ISO 6252 can be used as a surrogate, but correlation with bottle field performance is product-specific. Incoming resin lots should also be checked for melt flow rate according to ISO 1133-1, density according to ISO 1183-1, and visible contamination. A lot-to-lot MFR shift greater than ±0.05 g/10 min may indicate a supply-chain error or contamination and should be quarantined before use in blow moulding. Bulk density and pellet dimensions are less critical but should be recorded because changes in pellet geometry can affect solids conveying in grooved-barrel extruders.

When hot-filled closure systems are specified

For filling at product temperatures above 50 °C, the neck finish undergoes creep because it is less oriented than the blown sidewall. A closure applied at 60 °C can suffer back-off torque loss within 24 h if the finish shrinks or relaxes. The effect is stronger with polypropylene closures because the PP closure and HDPE neck have different creep rates. Top-load behaviour of the filled bottle should be evaluated according to ISO 12048. Neck diameter relaxation should be measured with a calibrated plug gauge after 24 h and 48 h at the intended storage temperature. Published data for this specific configuration is limited, and validation on the actual filling line is mandatory. Sharp edges on tamper-evident bands can initiate notches in the neck and should be examined before production.

For household bleach containing sodium hypochlorite up to 5 wt% at ambient temperature, HE2591 is suitable in standard bottle tests. Formulations containing dodecylbenzenesulfonate, nonylphenol ethoxylates, or terpenoid solvents are more aggressive and require ESCR testing in the fully formulated product. Ketones, aromatic chlorinated solvents, and strong swelling aliphatic hydrocarbons are outside the recommended continuous-contact range. The grade is not intended for injection moulding of closures, film, sheet, or pipe. Closure resins with MFR values above 2 g/10 min are outside the HE2591 processing envelope. For open-top drums above 60 L, lower-density high-ESCR HDPE grades are typically selected because wall-thickness and drop-impact requirements differ from small rigid bottles.

Compared with a lower-density HDPE blow moulding grade at 0.950 g/cm³, HE2591 provides higher top-load strength and better buckling resistance for a given wall thickness. Compared with a high-flow HDPE grade at 0.60 g/10 min, HE2591 has lower output on high-speed rotary wheel machines but better parison stability for containers above 1 L. Within the Borealis HDPE blow moulding portfolio, HE2591 is positioned for household chemical containers; grades intended for food contact or pharmaceutical packaging should be selected separately. Published comparative data under identical tooling and parison programming is limited, so a direct on-site comparison is required when replacing an existing resin.

Borealis supplies HE2591 with a safety data sheet and states that the grade is not classified as hazardous under Regulation (EC) No 1272/2008. The material is not intentionally formulated with substances of very high concern under REACH. For food-contact applications, the manufacturer generally does not market HE2591 as food-approved; a specific food-contact HDPE grade should be selected if compliance with Commission Regulation (EU) No 10/2011 is required. Mechanical recycling of HDPE containers is possible if residues of aggressive chemicals are removed before the bottle enters the recycle stream. These statements reflect published supplier documentation and require confirmation against the current safety data sheet.