ExxonMobil HDPE HMA-028 is classified as a high-density polyethylene grade engineered for high-molecular-weight blown film extrusion. The material is supplied in pellet form and is typically processed on grooved-feed, air-cooled or internal bubble-cooled blown film lines. Manufacturer-published typical values place the density at 0.958 g/cm³ when measured according to ASTM D1505 and the melt index at 0.28 g/10 min when determined at 190 °C/2.16 kg in accordance with ASTM D1238. These two coordinates situate the grade in the high-density, fractional-melt-index segment of the HDPE film portfolio, where melt strength and stiffness are prioritized over low-temperature dart impact relative to linear low-density polyethylene grades. The grade is not intended for cast-film or sheet-extrusion service; its molecular architecture is tailored for tubular film production and downstream conversion operations requiring dimensional stability and elevated modulus.

What Distinguishes the HMA-028 Molecular Architecture in Film Production?

The HMA series comprises high-molecular-weight HDPE film grades differentiated primarily by melt index, molecular weight distribution, and target gauge range. The 0.28 g/10 min melt index of HMA-028 is higher than that of HMA-016 and lower than general-purpose injection molding HDPE grades; this yields a balance between bubble stability and extruder motor load. The density of 0.958 g/cm³ places the material close to the upper end of the high-density specification range of 0.940–0.965 g/cm³ referenced in 21 CFR 177.1520(c). Published gel-permeation chromatography data for this specific grade are limited; however, the combination of fractional melt index and blown-film processing indicates a broad molecular weight distribution with a high-molecular-weight tail that contributes shear thinning and extensional viscosity. This molecular architecture differentiates HMA-028 from unimodal high-density grades used in cast film or sheet extrusion, where rapid relaxation and low die swelling are required. The broad distribution also affects processing: at equivalent melt index, a broad-distribution polymer can exhibit lower die-entry viscosity but longer relaxation time than a narrow-distribution metallocene resin.

Thermal and rheological boundaries should be established on the target extrusion line because the manufacturer issues only nominal melt index and density values without dynamic oscillatory rheology. Melt temperature at the die is commonly maintained between 190 °C and 220 °C for this molecular weight class; the lower bound limits melt fracture and die-lip deposit, while the upper bound prevents oxidative chain scission during residence times above 3 min. Extruder head pressure on a 90 mm grooved-feed blown film line with 30:1 L/D is typically higher than for an equivalent screw diameter on a smooth-bore line due to forced conveying of high-molecular-weight polymer. Operators should monitor specific energy input rather than barrel set temperatures alone; a sudden rise in melt pressure at constant screw speed after screen pack changes often indicates gel accumulation from degraded high-molecular-weight species. Pre-drying is not normally required for unopened, moisture-protected pellet deliveries unless ambient humidity exceeds 60% RH or storage has exposed pellets to condensation, in which case drying at 80 °C for 2 h in desiccant-bed dryers can reduce surface moisture to below 0.02 wt%.

Rheological Constraints on Grooved-Feed Blown Film Lines

High-molecular-weight HDPE such as HMA-028 exhibits pronounced non-Newtonian behavior. The shear viscosity decreases significantly with increasing shear rate in the die gap, which permits extrusion at commercial rates despite the low melt index. However, the relaxation time is long; melt exiting a die gap of 1.0–1.5 mm may retain molecular orientation if the blow-up ratio is below 2.5:1 or if the frost line height is shorter than 6 die diameters. Under these conditions, film can exhibit anisotropic shrinkage in the machine direction and increased dart impact asymmetry. Typical blown film parameters for high-molecular-weight HDPE include a blow-up ratio of 3:1 to 4:1, a die gap of 1.2–1.8 mm, and a frost line height of 8–12 die diameters. Melt temperatures at the die below 180 °C have been associated with melt fracture and cobweb-type defects on high-output lines; temperatures above 230 °C can destabilize the bubble as melt strength declines and cooling capacity becomes rate-limiting. The recommended melt temperature range for this molecular weight class is therefore 190–220 °C. Screw designs with moderate barrier sections and mixing pins are preferred over high-shear Maddock-type mixers because excessive shear heating can generate localized temperatures exceeding 250 °C despite lower barrel set-points.

Property cliff-edge behavior is observed when the melt index drifts outside the nominal range. If a lot exceeds 0.35 g/10 min under ASTM D1238, bubble stability and gauge control at thin gauge below 12 µm may deteriorate because extensional viscosity is insufficient to resist transverse gauge variation. If the melt index falls below 0.22 g/10 min, extruder melt pressure may exceed the safe operating limit of older 75 mm extruders rated for 350 bar, causing pressure-relief alarms and reduced output. Incoming lot inspection should verify melt index and density before silo blending. Off-spec lots should not be processed on retrofitted lines without recalibrating downstream nip speed and air-ring supply pressure. On lines fitted with internal bubble cooling and segmented air rings, bubble stability windows are generally wider; however, high-density high-molecular-weight grades develop crystalline orientation rapidly at the frost line. The frost line position should not be allowed to oscillate more than ±10 mm during a production run. Oscillation of the frost line is often the first visible indicator of melt temperature drift or ambient air fluctuation. Lip gaps at the die exit can be adjusted between 0.6 mm and 2.0 mm, but gaps above 2.0 mm reduce transverse direction orientation and lower film yield strength. Conversely, die gaps below 0.8 mm generate high shear rates above 1000 s⁻¹ and may trigger sharkskin surface defects if melt temperature is below 200 °C. Adjusting air-ring pressure and frost line height is more effective than raising temperature alone for eliminating optics defects such as die lines, orange peel, and bubble chatter.

Film tensile properties should be measured after conditioning for at least 40 h at 23 °C/50% RH in accordance with ASTM D618 procedures referenced by ASTM D882. Machine-direction tensile yield strength for high-density film at this density is typically in the range of 25–35 MPa; transverse-direction yield strength is commonly 5–10% lower due to blow-up ratio effects. Elongation at break often exceeds 600%, but this parameter is strongly gauge-dependent and should not be used as a batch release criterion without control charts. The flexural modulus reported for compression-molded specimens according to ASTM D790 is not directly equivalent to film stiffness; for film, secant modulus at 2% strain is the appropriate measure. Dart drop impact values from ASTM D1709 can be expected to fall below those of butene-based LLDPE film of identical gauge, but above typical unimodal HDPE cast film. Specific published values for HMA-028 depend on film thickness, blow-up ratio, frost line height, and downstream draw. It is therefore necessary to generate line-specific property curves when replacing LLDPE in a finished article.

When HMA-028 Replaces HMA-016 or HMA-018 in Thin-Gauge Stiff Film

The primary difference between HMA-028 and lower-melt-index grades in the HMA family is flow-induced orientation. HMA-016, with a typical melt index of 0.16 g/10 min, delivers higher melt strength and can be processed at wider die gaps and higher blow-up ratios, but it imposes higher extruder motor load and may reduce maximum output on grooved-feed lines with limited torque. HMA-028 increases melt index by approximately 75% relative to HMA-016, permitting faster screw recovery, lower head pressure, and thinner gauge capability. The trade-off is a measurable reduction in melt strength; therefore, downgauging from 20 µm to 12 µm may require a higher blow-up ratio, a shorter frost line, or internal bubble stabilizing equipment to avoid bubble flutter. When substituting HMA-028 for HMA-018, the change is less severe but still relevant for lines operating near a throughput of 500 kg/h; the higher melt index of HMA-028 can reduce melt pressure by an estimated 5–10%, but this estimate lacks published line-specific verification. Published data for this specific substitution configuration is limited; plant trials should map frost line and bubble diameter against target gauge variation before permanent recipe changes are made.

Typical end uses for HMA-028 include T-shirt grocery sacks, high-stiffness shopping bags, can liners, and multi-wall liners where film modulus and creep resistance are valued. In T-shirt grocery sacks, film produced at 15–18 µm thickness on high-speed conversion lines benefits from the grade’s density-driven stiffness, measured as secant modulus at 2% elongation per ASTM D882. The moisture vapor transmission rate decreases as density rises; barrier validation should be performed according to ASTM F1249 because orientation and gauge dominate measured values. Low-temperature ductility is more limited than for butene-based LLDPE, and applications requiring freezer impact resistance should be validated with ASTM D1709 dart drop testing using the actual film gauge. The narrow molecular weight distribution of metallocene LLDPE is not reproduced here; HMA-028’s broad distribution contributes lower overall toughness but higher modulus and better dimensional stability in converting operations. On bag-making systems with optical registration, film from HMA-028 may require adjustments to web tension because the modulus rise in the machine direction can alter draw characteristics at printing stations.

Managing Lot-to-Lot Variation in High-Molecular-Weight Film Grades

High-molecular-weight HDPE extrusion is energy-intensive. On a 120 mm grooved-feed extruder, specific energy input may exceed 0.25 kWh/kg at maximum output; barrel cooling zones often operate below 100 °C because forced conveying generates frictional heat. Melt pumps are less common on blown film lines but may be installed to damp pressure fluctuations; if a melt pump is used, pump inlet pressure should be maintained above 20 bar to avoid cavitation while suction pressure below 10 bar may produce discharge pressure instability. Screen packs of 40/60/100 mesh are typical; fine screens above 100 mesh increase melt temperature and pressure but reduce gels. Screen changes should be scheduled on the basis of pressure rise across the pack, not calendar hours; a pressure rise above 50 bar from the clean condition indicates accumulation of degraded polymer or contamination and warrants inspection.

The density of 0.958 g/cm³ corresponds to a crystalline fraction of approximately 75–80% based on the two-phase model using 1.00 g/cm³ and 0.855 g/cm³ as crystalline and amorphous limits, respectively. Rapid cooling in blown film suppresses secondary crystallization; subsequent post-extrusion shrinkage is therefore a processing variable. Film wound in rolls may shrink if exposed to storage temperatures above 40 °C, causing gauge bands and telescoped rolls. This is especially problematic for high-density grades with slower relaxation, and warehouse conditions should be monitored when film is destined for automated conversion lines with optical sensors.

Food-Contact and Regulatory Compliance Matrix

Operational boundaries include avoidance of prolonged storage in direct sunlight unless UV stabilizer masterbatch is added at letdown ratios consistent with the masterbatch supplier’s recommendation. HMA-028 should not be blended with polyamide or EVOH barrier resins without a tie-layer; direct co-extrusion of dissimilar polymers without an adhesive tie layer can produce delamination and unstable layer interfaces. When using recycled HMA-028 film regrind, addition levels above 20 wt% may increase gel counts and reduce dart impact because of oxidative chain scission after multiple heat histories. The grade is not recommended for sustained contact with strong oxidizing acids at temperatures above 60 °C, where stress-cracking resistance should be evaluated according to ASTM D1693 on finished articles. Published data for HMA-028 ESCR values in concentrated detergent environments is limited; field validation on formed parts is required before commercial use in rigid packaging or chemical containers.