LyondellBasell HDPE ALATHON H5618 is a high-molecular-weight high-density polyethylene copolymer supplied as pelletized resin. The grade is intended for extrusion blow molding of large rigid parts in which parison melt strength, environmental stress-cracking resistance, and lot-to-lot consistency are process-critical. Manufacturer documentation lists a density of 0.956 g/cm³ according to ASTM D1505 and a high-load melt index of 6.0 g/10 min according to ASTM D1238 at 190 °C/21.6 kg. The nominal 2.16 kg melt index is approximately 0.06 g/10 min, indicating the high viscosity required to prevent parison sag on large accumulator or continuous shuttle machines.

Primary application areas include 55-gallon industrial drums, agricultural chemical containers, intermediate bulk container liners, automotive fuel tanks, and large blow-molded industrial components. Finished-container performance is normally verified by top-load compression under ASTM D642, environmental stress-cracking under ASTM D1693, and drop impact under ASTM D2463. Published mechanical property data for the resin include a tensile yield strength in the range of 27–30 MPa under ASTM D638, elongation at break greater than 600%, and flexural modulus in the range of 1,300–1,400 MPa under ASTM D790. Low-temperature brittleness is reported below −75 °C when tested under ASTM D746. The ESCR value under ASTM D1693, 100% Igepal, F50 is product-release controlled; current lot data should be obtained because the result is sensitive to molded-in stress and test temperature.

Representative property values are summarized below; they are not to be used as purchase specifications.

Why Does High-Load Melt Index Govern Parison Behavior in Large-Part Blow Molding?

High-load melt index is measured at 190 °C/21.6 kg and reflects flow through a standardized capillary under elevated shear. The value is not a direct measure of low-shear melt strength; however, the ratio between the high-load melt index and the conventional melt index is used as an indirect indicator of molecular weight distribution. For Alathon H5618, the ratio of 6.0 g/10 min to 0.06 g/10 min is 100. A melt flow ratio in this range indicates a broad molecular weight distribution, which contributes to high melt strength and parison hang time. The high molecular weight tail increases die swell and reduces gravity-driven sag, allowing larger parison diameter and length to be extruded without unacceptable thinning.

In extrusion blow molding, the parison is extruded downward and must maintain dimensional stability until mold closure. For a given parison diameter and wall thickness, the elapsed time between extrusion and mold closure determines the minimum low-shear viscosity required. HDPE grades with a conventional melt index near 0.06 g/10 min are in the fractional-melt range and are therefore not suitable for fast injection cycles; they are selected for large-part blow molding because the high viscosity supports the parison mass. The use of a high-load melt index test resolves flow at practical shear rates because the 2.16 kg value may be too low to detect lot-to-lot differences in high molecular weight fractions.

Rheological measurements on high-molecular-weight HDPE show pronounced shear thinning; apparent viscosity decreases by an order of magnitude between 10 s⁻¹ and 1000 s⁻¹. This behavior permits extrusion through the die at moderate backpressure while retaining low-shear melt strength. The exact viscosity curve for H5618 should be obtained from capillary rheometry under ASTM D3835, because single-point melt index cannot capture curvature in the viscosity-shear rate profile.

High-molecular-weight HDPE grades with broad molecular weight distribution can exhibit melt fracture at high shear rates if the die gap is too narrow. In continuous extrusion blow molding, die gaps of 0.8–1.2 mm and moderate shear rates are used; if the die gap is reduced below 0.8 mm, surface shark-skin may appear. This is a process limitation, not a material failure. Production lines are therefore configured with polished die lips and temperature-controlled die heads.

Field data from production-scale accumulator-head machines confirm that parison sag is sensitive to melt temperature and die gap. When the die temperature is increased by 5 °C above the optimum range, a measurable increase in parison length variation occurs on parts with shot weights above 5 kg. This behavior is not unique to H5618 but is amplified by the broad molecular weight distribution. The processing window is therefore controlled within ±5 °C at the die to maintain consistent wall thickness and weld-line strength.

Extrusion blow molding operations running Alathon H5618 on a single-screw extruder with grooved feed section and barrel L/D ratio from 24:1 to 30:1 typically set barrel zones between 180 °C and 210 °C. The accumulator head and die are maintained near 200–220 °C. Melt temperature at the die should remain above 190 °C to prevent unmelted particles from producing parison weld lines and below 230 °C to limit oxidative chain scission. On machines equipped with a melt pump, pressure pulsation ahead of the die can be reduced to less than ±0.5 MPa, which improves parison length consistency. Screw torque is higher than for lower-molecular-weight HDPE grades; therefore, a high-torque gearbox and motor load monitoring are required for continuous production.

Typical blow mold temperature for HDPE large-part production is 10–25 °C. Lower mold temperatures shorten cooling time but may increase residual stress and warpage; higher mold temperatures improve surface gloss but reduce cycle rate. Blow air pressure is usually maintained between 0.5 MPa and 0.8 MPa to force the parison against the cavity during cooling. Effective parison programming with a diverging die is used to control wall thickness distribution; wall-thickness deviations are detected by section-weight analysis or ultrasonic gauging.

Regrind from trimmed parisons and rejects is reused in blow molding of high-molecular-weight HDPE. The allowable regrind fraction is established by finished-part testing under ASTM D1693, ASTM D642, and ASTM D2463 rather than by a fixed percentage. Moisture in the base pellet is typically low; however, if steam bubbles or visible splay appear in the parison, a desiccant hopper dryer set at 80 °C for 2 h may be applied to remove surface condensation from cold storage. Published data for drying requirements specific to Alathon H5618 is limited.

Property Thresholds Relevant to Industrial Container Service

In large industrial drums, top-load strength is governed by side-wall buckling and material modulus. The flexural modulus of Alathon H5618 in the range of 1,300–1,400 MPa under ASTM D790 supports high column strength, but final top-load performance depends on wall thickness, ribbing, and weld-line quality. Top-load failure on finished containers is evaluated under ASTM D642; a change in resin flexural modulus alone does not guarantee improved top-load rating because geometry and processing history dominate.

Environmental stress-cracking resistance is evaluated on compression-molded plaques under ASTM D1693, 100% Igepal, F50. For HDPE copolymers, ESCR is controlled by comonomer type, distribution, and molecular weight. Alathon H5618 is selected for chemical containers because its comonomer-modified structure provides higher ESCR than a homopolymer at equivalent density; however, the exact value is affected by test duration, plaque thickness, and degree of crystalline orientation. Users of the grade in containers carrying surfactants, agricultural chemicals, or alcohol-water mixtures should specify lot-specific ESCR data.

Low-temperature impact resistance is relevant for automotive fuel tanks and outdoor containers. The resin brittleness temperature is reported below −75 °C under ASTM D746, but component impact performance is better assessed by drop impact on the finished part. For blow-molded containers, ASTM D2463 provides a method for drop impact testing; for automotive fuel tanks, finished fuel tank impact and permeation testing is conducted according to OEM-specific procedures, and the resin cannot be certified solely by plaque data. Published data for H5618 in this specific configuration is limited.

When HDPE Components Are Transferred from Injection Molding to Extrusion Blow Molding

Alathon H5618 differs from injection molding HDPE grades in molecular weight, melt viscosity, and process speed. Injection molding HDPE grades typically show a 2.16 kg melt index above 5 g/10 min, which permits rapid mold filling through runners and gates. Alathon H5618 has a 2.16 kg melt index near 0.06 g/10 min; it is not suitable for thin-wall injection because high pressure drop and long filling time cause short shots and molded-in stress. The grade is supplied for extrusion blow molding, where high viscosity is an advantage for parison hang time.

Compared with lower-density HDPE blow molding grades in the 0.945–0.950 g/cm³ range, Alathon H5618 at 0.956 g/cm³ provides higher flexural modulus and top-load stiffness. The trade-off is that higher density can reduce ESCR if all other molecular parameters are equal. Compared with higher-density HDPE grades above 0.960 g/cm³, Alathon H5618 may retain better ESCR because of comonomer incorporation, but direct comparisons require paired plaques tested under ASTM D1693, 100% Igepal, F50. Within the Alathon grade family, published comparative data between H5618 and adjacent blow molding grades is limited; selection should be based on current datasheets and trials on the target machine.

When replacing a lower-viscosity blow molding HDPE with H5618, processors may need to increase barrel heater settings or reduce screw speed to avoid excessive melt temperature rise from shear heating. The reduction in output compared with a higher-HLMI grade is compensated by improved parison stability. Conversely, if H5618 is replaced by a grade with higher HLMI and lower molecular weight, wall thickness uniformity in large parts may deteriorate unless parison programming is changed.

For food-contact applications, compliance is governed by the testing and formulation conditions of the finished article. The base HDPE is typically used as a component of food-contact packaging that is evaluated under FDA 21 CFR 177.1520(c) and EU 10/2011 Annex I. The suitability of the final part depends on additives, colorants, process aids, and regrind history; therefore, the supplier certificate for Alathon H5618 does not by itself certify a finished container for food use. REACH compliance is assessed under EC 1907/2006, including Annex XVII restrictions, and RoHS compliance is evaluated under 2011/65/EU Annex II when the part is used in packaging for electrical and electronic equipment.

In continuous service, the material is constrained by creep and oxidation limits common to HDPE. Sustained loads at temperatures above 60 °C produce measurable creep; for structural parts, long-term modulus data generated under ISO 899-1 or ASTM D2990 should be applied. Direct contact with strong oxidizing acids, aromatic hydrocarbons, and chlorinated solvents at elevated temperatures can reduce ESCR and cause surface attack. These media should be evaluated by immersion testing under ASTM D543 before finished-part approval.