On landfill cell, embankment facing, and industrial containment pond projects, smooth geomembrane sheet is converted from Chevron Phillips Chemical Marlex HDPE 9398T on a calendered flat-die line rather than a blown film tower when thicknesses from 1.0 mm to 3.0 mm and roll widths above 5.0 m are specified. Virgin resin is air-conveyed to an indoor silo and gravimetrically dosed into a 120 mm single-screw extruder with a 30:1 L/D grooved-feed section and a barrier screw. A 50% carbon black masterbatch is added at 4.0–5.0 wt% to obtain a final carbon black content of 2.0–2.5 wt%. The melt passes a 60/80/100 mesh screen changer before entering a coat-hanger slot die set to a 2.5 mm lip gap. Calendering rolls then polish both surfaces at roll temperatures between 70°C and 90°C while drawing the sheet to final thickness. The nominal density of 0.939 g/cm³ is determined by ASTM D1505, and the nominal melt index of 0.8 g/10 min at 190°C/2.16 kg is determined by ASTM D1238. Those two properties provide melt strength during roll takeoff and sufficient low-shear flow for edge trim regrind. If silo storage dew point produces surface moisture at relative humidity above 60%, hopper drying at 80°C for 2–4 h is specified to prevent steam porosity in thick sheet. On construction sites, hot-wedge seam welding is run with a 400–450°C wedge surface temperature and a travel speed of 1.5–2.2 m/min. Seam peel and shear values are then verified by ASTM D6392. The primary processing defect is die-lip build-up at the sheet edges when regrind levels exceed 20 wt%; that condition creates local oxidation gels that fail seam continuity. Carbon black dispersion is checked per ISO 18553, because agglomerates above 30 µm act as stress concentrators in the finished liner. In addition, silo cross-contamination with polypropylene at 0.5 wt% or greater produces discrete unmelded PP inclusions at HDPE melt temperatures, and those inclusions are initiation sites for brittle weld separation.

What Limits Slow Crack Growth in Corrugated Drainage Pipe Made from 9398T?

In drainage-grade HDPE pipe extrusion, corrugated profile wall is formed on a vacuum-forming corrugator in which the parison passes between continuously advancing mold blocks. A 60 mm grooved-feed extruder with a 30:1 L/D screw melts the resin at 190–220°C and pumps it through a 60/80/100 mesh screen pack. The melt is then distributed through an annular die before entering the mold blocks. Carbon black is introduced as a 50% masterbatch at 4.0–5.0 wt% to meet outdoor UV stabilization requirements. The critical performance parameter is not short-term burst strength but slow crack growth resistance when the pipe is backfilled under point loads. Hydrostatic design basis for HDPE pipe is evaluated under ASTM D2837. Stress crack resistance under service conditions is assessed with the notched pipe test ISO 13479 or the Pennsylvania Edge Notch Test ASTM F1473. Field failures on corrugated drainage pipe are commonly traced to scoring on the inner wall during final cutting. Grooves deeper than 0.3 mm can become crack initiation points if the resin lacks sufficient high-molecular-mass fraction. Converters using 9398T therefore specify a melt temperature no higher than 220°C to avoid reducing the high-molecular-weight tail. It is equally important to avoid regrind ratios above 25 wt%; repeated extrusion at high shear lowers notched slow crack growth performance. Finished pipe is tested for flattening at 20°C and 60°C per ASTM F894. Stub-and-socket joints are leak-tested at 10 kPa internal pressure for 10 min in drainage specifications.

Commercially, extrusion blow molding with Marlex HDPE 9398T is used for 220 L open-top drums, 1,000 L intermediate bulk containers, and industrial chemical storage units where UN 1H1 certification and resistance to environmental stress cracking are specified. The accumulator head delivers shot weights from 5 kg to 20 kg at melt temperatures of 190–210°C. The die gap is set at 2.0–3.5 mm to control parison swell and wall thickness. A core conflict in this process is between parison sag resistance, which requires high melt strength and lower melt temperature, and knit-line integrity around the pinch-off zone, which requires adequate melt flow and higher temperature. The broad molecular weight distribution of 9398T permits a bounded correction. Melt temperature should not be dropped below 185°C because the pinch-off weld becomes visually rough and impact failures appear under UN drop testing at -18°C. Molds are run with clamp forces between 200 t and 600 t depending on projected area and pinch-off length. ESCR is checked on molded plaques under ASTM D1693 Condition A or B with Igepal CO-630 at 50°C. Finished drums are subjected to the UN 1H1 sequence of drop, leakproofness, hydraulic pressure, and stack tests. Hydraulic test pressure is typically 100 kPa for 30 min. A frequent production failure appears when flash trim is recycled above 30 wt%: the melt becomes too low in extrudate swell, causing thin sidewalls below the 1.8 mm minimum wall for dangerous goods containers. Surface fluorination may be applied to reduce solvent permeation for containers holding aromatic hydrocarbons. The treatment is applied at 1.0–2.0 vol% fluorine in nitrogen for 10–30 min, followed by venting to residual fluorine below 1 ppm before personnel access.

Blown Film Bubble Chemistry and Frost Line Control

On a spiral mandrel die line with a 350 mm die diameter and a die gap of 0.8–1.2 mm, heavy-duty HDPE blown film based on 9398T is produced using a 90 mm extruder with a 30:1 L/D barrier screw. The grade is processed in a stalk configuration rather than a pocket configuration because the high-molecular-weight fraction stabilizes the bubble between the die and the frost line. Blow-up ratios from 2.0:1 to 4.0:1 are used. At higher ratios, bubble flapping and edge curl increase. The extrusion melt temperature is held between 190°C and 210°C, and the frost line is positioned at 8–10 die diameters above the air ring. Tensile yield and elongation are measured per ASTM D882, dart impact per ASTM D1709 Method A, and Elmendorf tear per ASTM D1922. The main property conflict is machine-direction tear versus transverse-direction dart impact. Increasing blow-up ratio and takeoff speed raises orientation in the machine direction, which improves tensile strength but causes brittle tear in the machine direction. Converters therefore lower takeoff speed or reduce the stalk height to rebalance orientation. Film gauge is commonly 50–150 µm for industrial covers, construction debris containment, and outdoor temporary enclosures. At thicknesses below 30 µm, dart impact values fall below typical site specifications, and pinhole counts increase when melt temperature exceeds 210°C due to oxidation gels. The resin is also blended with 5–10 wt% low-density polyethylene in some converters’ formulations to improve bubble stability and dart impact. That blend requires downward adjustment of the frost line to avoid excessive blocking on the collapsing frame.

When HDPE 9398T Replaces Steel in Chemical Storage Liners, Weld Integrity Dictates Service Life

In fabricated chemical storage liners, sump basins, and secondary containment pans, sheet sections from 3.0 mm to 12.0 mm are cut and butt-welded with a single-screw extrusion welder using a 4.0 mm shoe and a weld gas preheat temperature of 250–300°C. The welding rod is produced from the same resin or from a closely matched HDPE. Rod consumption is calculated to leave a 2.0–3.0 mm weld bead above the parent sheet. Tensile testing of welds is performed per DVS 2207 or ASTM D638 Type IV specimens. Elongation at break is typically lower than the parent sheet by 10–20%. Chemical immersion resistance is screened under ASTM D543 for 7-day and 30-day exposure. ESCR after immersion is assessed under ASTM D1693. A documented limitation for HDPE liners is continuous contact with strong oxidizing acids above 60°C; the oxidative attack reduces oxidative induction time and accelerates weld-edge cracking. Aromatic hydrocarbons, ketones, and chlorinated solvents plasticize the amorphous fraction and lower service temperature limits. Design stress should be reduced by 50% when the stored liquid is a known stress crack agent. The liner is vacuum-box tested at 30 kPa negative pressure over all weld seams to detect through-leaks before hydrostatic testing. In secondary containment, the liner is often installed over a geotextile cushion and welded to HDPE pipe penetrations using a combination of extrusion and hot-wedge welds. The most frequent failure is not chemical degradation but weld root notch formation from over-grinding the weld bead. For that reason, grinding depth into the parent sheet is limited to 0.2 mm maximum. Direct contact with copper piping in wet service should be avoided because copper ions catalyze thermo-oxidative degradation of the polyethylene matrix.

For industrial dunnage and vehicle body liners, 9398T sheet is extruded in thicknesses between 6.0 mm and 25.0 mm, then CNC-routed, drilled, or post-formed. The sheet is run on a single-screw or twin-screw line with a flat die and three-roll stack. Roll temperatures of 70–90°C are used to control surface gloss and residual stress. Test requirements for this application include tensile yield strength per ASTM D638, flexural modulus per ISO 178 or ASTM D790, and notched Izod impact per ASTM D256 at 23°C and -40°C. Because HDPE has a coefficient of linear thermal expansion of approximately 120–180 µm/m/°C, mounting holes for large panels must allow for expansion; otherwise buckling occurs in service. The lower density of 0.939 g/cm³ provides a mass reduction relative to steel sheet of roughly 8× on a thickness-equivalent basis. A process constraint is the slow cooling of thick sheet in the center. If the sheet is cut immediately after rolling, internal thermal stresses release during machining and cause dimensional drift greater than 0.5 mm/m. Sheets are therefore racked and air-cooled for 24 h minimum before routing. Flame-cutting is not recommended because HDPE ignites at elevated autoignition temperatures and burning drips present a severe fire hazard. Sawing with carbide-tipped blades at 3,000–4,000 rpm and compressed-air cooling avoids gumming.