Chevron Phillips Chemical HDPE 9518H is a high-density polyethylene grade supplied under the Marlex resin series by Chevron Phillips Chemical. The grade designation identifies a high-molecular-weight, fractional-melt resin intended for sheet extrusion, thermoforming, and large-part blow molding. Representative datasheet values place the nominal density at 0.954 g/cm³ when tested under ASTM D1505, with a melt index of 0.18 g/10 min at 190 °C and 2.16 kg load under ASTM D1238. The high-load melt index is 18 g/10 min at 190 °C and 21.6 kg load, giving a melt flow ratio of 100. This ratio is not a specification but serves as a rheological indicator of a broad molecular weight distribution. The grade is described in supplier literature as an ethylene-hexene copolymer, with the comonomer used to increase stress crack resistance while maintaining density above 0.950 g/cm³.

The low-load melt index of 0.18 g/10 min places 9518H in the high-molecular-weight HDPE class. Such grades are not used in injection molding or thin-wall blow molding because the low flow under 2.16 kg load increases fill pressure and may cause short shots. In sheet extrusion, the same low flow improves melt stability and permits thicker free-standing sheet without excessive sag. The high-load melt index of 18 g/10 min indicates that under a 21.6 kg load the material flows at a rate 100 times the low-load value. This shear sensitivity is the primary process distinction from narrow-molecular-weight HDPE grades of similar density, which may have melt flow ratios from 50 to 70. Published data for the exact molecular weight distribution of 9518H is limited; the melt flow ratio is the available indirect indicator.

What Distinguishes 9518H from Conventional HDPE Blow Molding Grades?

Lower-viscosity HDPE blow molding resins with melt indices of 0.30 g/10 min to 0.45 g/10 min are typically selected for small bottles and thin-wall containers. When those resins are used for large parts, parison sag can produce nonuniform wall thickness and weld-line weakness. 9518H differs by shifting the low-load melt index downward to 0.18 g/10 min while maintaining sufficient high-load flow for extrusion. In continuous-extrusion blow molding, the lower sag rate allows longer parison hang time and larger shot weights without crosslinking. In accumulator-head blow molding, the same property allows the molten accumulator charge to be displaced without excessive head pressure. The density of 0.954 g/cm³ is near the upper end of the HDPE density range for large-part blow molding, providing stiffness without the rapid notch sensitivity of high-density homopolymers.

Mechanically, the grade differs from commodity HDPE sheet resins because the lower melt index and broader molecular weight distribution contribute to higher melt strength and less sheet thinning during deep-draw thermoforming. Representative supplier values for tensile yield strength and flexural modulus are 26 MPa and 1,100 MPa when tested to ASTM D638 and ASTM D790, respectively. Elongation at break is reported above 600%, indicating ductile deformation under slow loading. These values are typical rather than lot-release limits; converters must request current certificates of analysis for lot-specific data. The notched Izod impact value under ASTM D256 and environmental stress crack resistance data under ASTM D1693 are not fixed by the density and melt index alone; they depend on comonomer type, catalyst residues, and sheet cooling history.

Representative datasheet values for Chevron Phillips Chemical HDPE 9518H are shown below. The values should be treated as typical and not as specification limits.

Extrusion, Thermoforming, and Roll-Stack Conditions for Fractional-Melt HDPE Sheet

Production-scale sheet extrusion of 9518H is typically performed on single-screw extruders from 65 mm to 120 mm with 30:1 L/D to 36:1 L/D. A barrier screw with mixing pins or a Maddock section is used to disperse the high molecular weight fraction without excessive shear heating. Melt temperature at the die is maintained between 204 °C and 218 °C. Operation above 221 °C can increase edge tear and surface oxidation; operation below 199 °C can produce high back pressure and poor melt homogenization. A screen pack with 80 mesh to 120 mesh is standard, and back pressure is typically kept between 5 MPa and 25 MPa at the screw tip. The die gap is set 10% to 20% wider than the target sheet thickness because the melt relaxes after the die lips.

For sheet from 3 mm to 12 mm, a three-roll stack with polished or matte rolls is operated at 75 °C to 95 °C. The roll gap controls gauge without overcalendering; excessive nip pressure can induce anisotropic shrinkage after reheating. Sheet surface temperature for thermoforming is normally 165 °C to 177 °C. Uniform core heating is required because the high-molecular-weight fraction resists sheet sag and can hide inadequate heating. Plug-assisted vacuum forming with aluminum tooling at 30 °C to 60 °C is used for draw ratios above 1.5:1. Sheet thickness variation should be held below 5% to reduce thin-out at corners. Published data for the exact sag rate of 9518H at specific sheet temperatures is limited; line trials are required to set oven dwell time.

Compared to lower-molecular-weight HDPE sheet grades, 9518H requires longer oven dwell times because the high-molecular-weight fraction retains melt elasticity. In sheet thermoforming, surface temperature alone can be misleading; infrared pyrometers may read 170 °C while the sheet core remains below forming temperature. Processors mitigate this by using twin-sided heaters and by reducing line speed rather than increasing zone setpoints. Overheating above 177 °C produces blistering and a loss of melt strength; underheating below 160 °C produces corner thinning and part warpage after demolding.

In blow molding, the grade is processed on accumulator-head machines or continuous-extrusion shuttle machines. For large parts from 5 kg to 30 kg, accumulator capacity is typically selected so the shot is 70% to 85% of total capacity; this avoids bridging and reduces residence time in the head. The melt temperature in the head is held between 205 °C and 218 °C. Blow pressure of 0.6 MPa to 0.9 MPa is common for large industrial parts. The parison programmer should be used to thicken the pinch-off and corner regions; without programmed parison wall thickness, sidewall thinning at the part periphery becomes the limiting factor. The lower sag rate of 9518H permits a wider drop-time window than lower-molecular-weight HDPE grades, but die drool and stringiness increase if the head temperature exceeds 220 °C.

Field applications include industrial dunnage, agricultural chemical storage, recreational vehicle side panels, battery trays, and large equipment housings. These applications are selected based on stiffness, low-temperature impact, and chemical contact. The grade is not recommended for thin-wall containers below 500 mL capacity because the flow path length and low melt index create excessive pressure drop in small blow molds. The final part performance must be validated under the end-use chemical and temperature conditions; no single datasheet value replaces a part-specific validation.

When Regrind Recovery and Mixed-Polymer Contamination Create Process Boundaries

HDPE 9518H is re-processable in closed-loop post-industrial scrap, but each heat history shifts the melt index upward through chain scission. The broad molecular weight distribution of the virgin resin does not eliminate this shift. In sheet extrusion, post-industrial trim at 20 wt% to 30 wt% may produce a measurable increase in melt index and a reduction in melt strength; the exact shift is not fixed because it depends on screw shear, residence time, and melt temperature. Regrind should be ground from clean trim and stored indoors to avoid dust and moisture. Processors should monitor melt index shift according to ASTM D1238; a shift from 0.18 g/10 min to above 0.25 g/10 min indicates excessive thermal degradation and requires adjustment of screw speed or barrel profile.

Mixed-polymer contamination creates more serious process boundaries. Polypropylene scrap must be kept out of the HDPE regrind stream. In the melt, HDPE and polypropylene are immiscible, and even low levels of PP contamination can reduce weld-line strength in blow molding and produce delamination in thermoformed sheet. The incoming regrind should be identified by FTIR or differential scanning calorimetry when mixed-polymer bins are used. In sheet extrusion, contamination often appears as directional tear along the machine direction and variable gloss bands. These defects cannot be corrected by raising melt temperature; raising temperature above 221 °C may increase degradation without dispersing the incompatible phase.

HDPE absorbs minimal moisture under normal storage conditions, so 9518H does not require routine pre-drying. Surface condensation is the more likely moisture source. When pellets are moved from cold storage into a warm processing area at relative humidity above 60%, surface moisture can create splay in sheet and blow mold parts. In that condition, a 2 h drying step at 80 °C using hot-air or desiccant drying is applied. Temperatures above 90 °C should be avoided for prolonged periods because pellet surface softening can cause bridging in the feed throat.

Chemical resistance and environmental stress crack resistance are application-specific. Under ASTM D1693 testing with 100% Igepal CO-630 at 50 °C, high-molecular-weight HDPE of this density and comonomer class often exceeds 100 h; however, this value is not a supplier specification and must be confirmed against the current technical data package for 9518H. The resin is generally resistant to dilute acids, alkalis, and polar organic solvents, but strong oxidizing acids, chlorinated solvents, and aromatic hydrocarbons can soften or stress-crack HDPE. Chemical compatibility must be tested with the actual formulation and part stress level.

Thermoformed agricultural equipment covers are manufactured from 9518H sheet by heating to 170 °C and forming over aluminum tooling. The sheet is clamped while above 160 °C to prevent edge tear; clamping pressure is distributed across the frame to avoid local thinning. Parts are cooled in the mold until the surface temperature drops below 80 °C before demolding. Demolding above that temperature can produce warpage because the high-molecular-weight fraction retains heat in the part core. In this application, the selection of 9518H over lower-molecular-weight sheet grades is based on the combination of stiffness, sag resistance during heating, and low-temperature impact in the finished part.