Chevron Phillips Chemical HDPE HHM 5502-01ST, part of the Marlex HDPE series, is a high-molecular-weight hexene-copolymer high-density polyethylene engineered for blow molding and thick-section extrusion applications in which environmental stress crack resistance, melt strength, and parison stability govern production yield. The manufacturer specifies a nominal density of 0.955 g/cm³ under ASTM D1505 and a nominal melt flow rate of 0.35 g/10 min under ASTM D1238 at 190 °C/2.16 kg. The high-load melt index is typically reported in the 30–35 g/10 min range at 190 °C/21.6 kg, producing a flow ratio near 90:1. This melt-viscosity envelope provides sufficient extensional viscosity for extended parison hang time while retaining the ability to fill long-flow thick-section molds. The “01ST” suffix does not signal a change in base resin composition but identifies the manufacturer’s specific additive and stabilization package; the product remains within the HHM 5502 family, and direct published property differentials against non-ST variants are limited, so certificates of analysis should be reviewed for lot-level confirmation.

The “HHM” designation places the material in the hexene-copolymer high-density polyethylene class, distinguishing it from butene-based high-density grades and from homopolymer resins with similar density but lower environmental stress crack resistance. The hexene branch length contributes to tie-molecule formation in the interlamellar region, which is critical for slow crack growth resistance. Compared with a homopolymer of identical melt flow rate, the hexene copolymer typically shows higher notched ESCR under ASTM D1693 and a lower brittle transition tendency, although tensile yield stress and flexural modulus may be slightly reduced relative to a 0.960 g/cm³ homopolymer segment.

What Distinguishes a Hexene-Copolymer HDPE with a 0.35 Melt Flow Rate from the Homopolymer Segment?

At a constant density of 0.955 g/cm³, the selection of hexene as the comonomer changes the length of short-chain branches and the resulting tie-molecule population. Homopolymer HDPE of equivalent melt index tends to exhibit higher crystallinity and stiffness, but its environmental stress crack resistance falls rapidly when tested under ASTM D1693 Condition A or B with a notched specimen. The hexene-copolymer architecture shifts the failure mechanism from brittle craze propagation to ductile fibrillation, extending the F50 time before slow crack initiation. This shift does not appear as a sharp step change but as a nonlinear response to comonomer distribution and molecular weight tail; a broader high-molecular-weight fraction sustains longer parison integrity in accumulator-head blow molding and supports thicker wall sections in large containers.

The melt flow rate of 0.35 g/10 min is located at the low end of the HDPE blow molding window. It yields higher viscosity and higher shear heating than fractional-melt film grades but lower screw torque than pipe grades with melt flow rates below 0.10 g/10 min. The grade therefore occupies a practical middle ground: it extrudes on standard 24:1 to 30:1 grooved-barrel single-screw extruders without requiring a barrier screw, yet retains sufficient zero-shear viscosity to support parison lengths above 1.2 m on industrial accumulator machines. The die swell is relatively high, typically requiring die gap settings 1.5–2.0 mm greater than the target wall thickness, depending on shot size and melt temperature.

Processing on continuous shuttle blow molding equipment is governed by melt temperature, mold temperature, extrusion rate, and accumulator fill time. A starting melt temperature of 200 °C is common for the product class, with the allowable range from 190 °C to 230 °C. Operation below 190 °C increases melt viscosity and can produce chestnut-like surface roughness on the parison, particularly in thin sections. Operation above 230 °C for prolonged residence times accelerates thermo-oxidative degradation and reduces molecular weight, which is observed as a decrease in parison hang time and a shift in apparent melt flow rate measured on retained samples. The mold temperature for blow molded parts should be maintained between 10 °C and 30 °C to control cooling-induced shrinkage and top-load strength; for large industrial containers, a chiller with 5–10 °C supply water temperature is often used to keep cycle times within the economic range.

Moisture uptake in high-density polyethylene pellets is generally low, but surface condensation in silos or gaylord boxes can introduce volatiles into the melt. If hopper moisture exceeds 0.1 wt%, a drying step at 80 °C for 2 h is recommended before extrusion. Failure to remove surface moisture is typically visible as micro-porosity in the weld line area, reduced burst-strength reproducibility, and occasional parison splitting at the die exit. These effects are not unique to HHM 5502-01ST, but they are amplified in thick-section blow molding because the low surface-to-volume ratio slows devolatilization before the parison is inflated.

Batch-to-batch drift in melt flow rate does not normally require reformulation; however, a shift of ±0.03 g/10 min from the nominal 0.35 g/10 min should trigger an extrusion profile adjustment of 5–10 °C or a screw speed adjustment to maintain targeted melt pressure. On accumulator-head machines, melt pressure instability above ±10 bar at the die entrance can indicate a worn check-ring or mismatched barrel temperature profile rather than resin inconsistency; the grade’s relatively high molecular weight makes it sensitive to feed-throat temperature control and to the quality of the die-head temperature zones.

Melt index, density, and flow ratio are not independent control variables in this product class.

For HHM 5502-01ST, the nominal density of 0.955 g/cm³ is approximately 0.005 g/cm³ below a conventional HDPE homopolymer, but the presence of hexene comonomer changes crystallinity more than the density difference alone would suggest. Crystallinity governs modulus and chemical permeation resistance, while the amorphous fraction governs impact absorption and ESCR. A higher-density homopolymer may exhibit higher flexural modulus, but it sacrifices the tie-molecule population required for slow crack growth resistance in large containers. Conversely, lowering density below 0.950 g/cm³ would improve ESCR further but reduce top-load strength and increase permeability, which is undesirable in automotive fuel tank barrier applications. The 0.955 g/cm³ target is therefore a balancing point for stiffness, ESCR, and permeation resistance.

The melt flow rate is measured under ASTM D1238 conditions 190 °C/2.16 kg; the ISO equivalent is ISO 1133-1:2022. The standard test reports a mass-flow rate, not a true viscosity, and should not be interpreted as a complete rheological fingerprint. The grade’s high-load melt index under 21.6 kg is used to calculate the flow ratio; a value near 90:1 indicates shear thinning that is stronger than that of unimodal injection molding grades but lower than some bimodal pipe grades. This shear thinning is beneficial for high-shear extrusion through narrow die gaps but must be managed to avoid excessive shear heating and potential shark-skin melt fracture at the die lip. Die-lip temperatures should be kept within 10 °C of the adapter temperature to minimize localized shear-induced degradation.

The apparent melt viscosity of a 0.35 g/10 min HDPE at 200 °C and shear rates typical of blow molding die lips, 100–1000 s⁻¹, is lower than the zero-shear viscosity by an order of magnitude. This shear thinning is a function of the molecular weight distribution and the high-molecular-weight tail. A narrow-distribution resin of identical melt flow would exhibit less shear thinning and generate higher die pressure at equivalent output, whereas a broader-distribution resin improves processability but may show slightly lower ESCR because the low-molecular-weight fraction contributes to craze initiation. The HHM 5502 series is positioned to balance these two effects; a drop in high-load melt index below 25 g/10 min would indicate a significant shift toward the high-viscosity end and would require revalidation of the accumulator head and die-lip setup.

The environmental stress crack resistance of HHM 5502-01ST arises from a combination of short-chain branch length, broad molecular weight distribution, and adequate tie-molecule density. In the standard constant-strain ESCR test under ASTM D1693, a notched specimen is immersed in a surfactant solution and held under a controlled bending strain; failures are reported as F50 times. The test is nonlinear with respect to density and melt index. A reduction in melt index from 0.35 g/10 min to 0.20 g/10 min generally improves ESCR by increasing the concentration of long chains that bridge adjacent lamellae, but it also raises extruder torque. Increasing density from 0.955 g/cm³ to 0.960 g/cm³ reduces the interlamellar amorphous thickness and can reduce F50 by an order of magnitude in some HDPE families. The hexene comonomer at this density moderates brittleness by creating a more uniform short-chain branch distribution compared with butene, which is less efficient at generating load-bearing tie molecules per unit comonomer content.

Slow crack growth in the field is not directly predicted by ASTM D1693 alone; notched constant tensile load tests, Pennsylvania notch testing, and full-scale container burst tests provide complementary data. In blow molded parts, weld lines and pinch-off regions are the most frequent initiation sites. The grade’s high molecular weight tail helps the parison compress at the pinch-off zone without severe thinning, but excessive mold closing pressure or insufficient pinch-off bar temperature can still produce a weak seam. Production-scale data indicate that weld-line thickness should be maintained at ≥85% of nominal wall thickness to avoid preferential crack initiation; below this threshold, ESCR values measured on compression molded plaques lose predictive validity.

The testing of ESCR should always be performed on compression molded plaques prepared according to the method referenced in the data sheet, because blow molded surfaces include orientation and cooling stresses that can either increase or decrease resistance depending on processing direction. If a laboratory reports an F50 below 100 h for a 0.955 g/cm³ hexene-copolymer grade of this melt index, the cause is often inadequate specimen conditioning or an incorrectly compounded test surfactant rather than a fundamental resin deficiency. Users should verify the lot-specific density and melt flow before interpreting an ESCR result; a density shift of only 0.001 g/cm³ can affect the test outcome.

When the 01ST Stabilization Package Is Subjected to Long-Term Thermo-Oxidative Load

The “01ST” additive formulation is selected to protect the polymer during pellet drying, melt processing, and end-use thermal exposure. The stabilization package includes a primary antioxidant and a secondary phosphite processing stabilizer, though the exact chemical registry and loading are not disclosed on public datasheets. The effectiveness of the package is measured by oxidation induction time under ISO 11357-6 or ASTM D3895; longer induction times correlate with a higher resistance to molecular weight loss during multiple heat histories. For containers exposed to repeated high-temperature washing or hot-fill operations, the additive package must not be prematurely consumed by residual chlorine or transition-metal ions from process water. A limit of ≤0.2 ppm free chlorine in hot-fill water is commonly imposed in production specifications, but the specific limit for HHM 5502-01ST should be confirmed with the manufacturer.

The resin is not inherently UV-stable. Outdoor service requires carbon black or an additional UV stabilizer masterbatch; without this, surface chalking and crackling occur within 6–12 months of direct sunlight in exposed climates. The grade should not be stored in direct sunlight or in silos with sustained temperatures above 50 °C. Strong oxidizing chemicals, including concentrated nitric acid above 20 wt%, can attack the polymer even if the antioxidant package remains active; the resin is also not recommended for continuous service above 80 °C under sustained mechanical load because creep and slow crack propagation accelerate. These limitations are consistent with the broader HDPE class and should be addressed during application qualification.

Industrial applications for HHM 5502-01ST include blow molded multilayer automotive fuel tanks, intermediate bulk container liners, 20–60 L UN-certified tight-head drums, agricultural chemical containers, and large recreational or industrial parts that require high ESCR. The hexene-copolymer architecture and 0.35 g/10 min melt index make the grade especially suited to accumulator-head machines producing heavy-shot parts with wall thicknesses above 4 mm. In fuel tank coextrusion, the grade is typically used as the virgin HDPE layer in combination with an ethylene-vinyl alcohol barrier layer and regenerated regrind; the viscosity of the 01ST grade is high enough to support the parison during multilayer die swell, but the presence of heat-sensitive barrier materials imposes a narrower melt temperature window of 190–220 °C to prevent scorch and black speck formation.

For thick-wall sheet and thermoforming applications, the grade can be processed on a single-screw extruder with a coathanger die and a three-roll polishing stack. The roll temperatures should be 60–80 °C to reduce frozen-in stress; sheet gauges from 6 mm to 25 mm are common. The melt strength of HHM 5502-01ST helps maintain sheet width stability and reduces draw sag, but the high molecular weight also reduces maximum screw throughput relative to lower-viscosity HDPE grades. On a 90 mm extruder with L/D 30:1, typical throughput rates in this product class are limited by melt temperature rise rather than screw capacity; increasing screw speed above 80 rpm can push melt temperature beyond 230 °C and is not recommended without a melt cooler or external temperature control.

Typical yield tensile strength for a 0.955 g/cm³ hexene-copolymer HDPE of this melt index is approximately 26–28 MPa under ASTM D638-14 Type IV specimens; flexural modulus is approximately 1,200–1,450 MPa under ASTM D790. These values are not specification limits and vary with specimen preparation. The combination of stiffness and ESCR makes the grade suitable for large containers where top-load and handling impact are simultaneous requirements. Lower-density hexene copolymers in the 0.945–0.950 g/cm³ range provide higher ESCR but lower top-load and higher permeation; higher-density homopolymers provide higher top-load but suffer from rapid crack growth at weld lines.

Comparative positioning against other high-density polyethylene grades

Relative to a general-purpose injection molding HDPE with a melt flow rate of 20 g/10 min, HHM 5502-01ST has much higher viscosity and is not suitable for thin-wall injection molding. The low melt flow permits the formation of stable parisons and thick sections, but it also increases injection pressure requirements and limits flow length-to-wall thickness ratios. If the same resin were used in an injection mold with a wall thickness below 1.5 mm, short shots and weld-line weakness would be expected. Conversely, replacing the product with a high-flow injection grade in a large blow molding application would cause parison draw-down, nonuniform wall distribution, and reduced pinch-off integrity.

Compared with a lower-molecular-weight HDPE film grade or a high-speed small-container blow molding grade with melt flow rates near 2–8 g/10 min, the HHM 5502-01ST delivers better ESCR but lower output. The grade is too viscous for high-output reciprocating blow molding tools designed for gallons of milk or thin-walled detergent bottles. Compared with a bimodal pipe-grade HDPE with melt flow rate below 0.10 g/10 min, the product offers easier extrusion and shorter cycle times but lower long-term hydrostatic strength; it is not specified for pressure-pipe service and should not be substituted into pipe applications requiring ISO 4427 or ASTM D2513 performance. Within the Chevron Phillips HDPE portfolio, the “HHM 5502” family spans a narrow melt index and density range; the “01ST” variant is intended where the manufacturer’s additive package for thermal stability and lot-to-lot consistency is required, but direct quantitative differentiation from other 5502 variants is not fully disclosed in public literature.

Chemical resistance of HDPE HHM 5502-01ST is consistent with high-density polyethylene: it resists most dilute acids, alkalis, and aqueous salt solutions up to 60 °C, but aliphatic and aromatic hydrocarbons cause swelling and a reduction in tensile strength. For agricultural chemical containers, the resin must be tested with the specific formulation because surfactants and solvents can reduce ESCR by plasticizing the amorphous regions. A container designed for 2 years of outdoor storage under stress with a concentrated surfactant should be evaluated under worst-case temperature rather than assuming ASTM D1693 data alone will predict field performance.

Regulatory compliance for HHM 5502-01ST is generally consistent with high-density polyethylene olefin polymers under FDA 21 CFR 177.1520, but the end-use article must meet migration testing requirements for specific food types and use conditions. The product is not automatically food-contact compliant unless the entire conversion process and additive package are considered. No specific FDA Food Contact Notification number is assumed. For the European Union, compliance with Regulation (EU) No 10/2011 on plastic food contact materials must be verified against the manufacturer’s declaration of compliance and migration limits; the product may also be covered by a REACH registration dossier, and the grade does not intentionally contain substances above the 0.1 wt% threshold listed in the SVHC candidate list. In electrical and electronic equipment, the polymer itself is not expected to contain restricted substances under RoHS Directive 2011/65/EU, but additives and color concentrates added downstream must be assessed separately.

Certificates of analysis for HHM 5502-01ST typically report lot-specific density, melt flow rate, and antioxidant package indicators rather than the complete mechanical property suite. For applications requiring Charpy impact under ISO 179-1 or notched Izod under ASTM D256, values should be generated from specimens taken from production tooling because laboratory compression molded plaques do not reproduce the orientation and cooling history of blow molded parts. If a specification requires flexural modulus under ASTM D790 or ISO 178, the test should be carried out at a defined thickness and conditioning time under 23 °C/50% RH; otherwise interlaboratory variation can exceed 5–7%.