Dow HDPE DGDB-5004 NT 7 is an unpigmented high-density polyethylene supplied in pellet form for extrusion blow molding of thick-walled industrial hollow parts. The NT 7 suffix denotes a natural grade supplied without added colour concentrate. Nominal density measured at 23 °C according to ASTM D1505 is 0.955 g/cm³. Nominal melt index measured at 190 °C under a 2.16 kg load according to ASTM D1238 is 0.85 g/10 min. These two values place the material in a low-melt-index, medium-high-density HDPE band used when parison melt stability and top-load stiffness must be balanced against flow length and thin-wall fill. Manufacturer-published typical values include tensile stress at yield of 28 MPa according to ASTM D638 Type IV, tensile elongation at break above 600% according to ASTM D638, 2% secant flexural modulus of 1,200 MPa according to ASTM D790, Vicat softening temperature of 127 °C according to ASTM D1525, and environmental stress crack resistance above 600 h under ASTM D1693 condition B with 10% Igepal CO-630 at 50 °C. These data are typical lot-average values, not specification limits, and should be verified against the current supplier certificate of analysis.

Values are supplier-published typical data for natural grade at 23 °C and 50% relative humidity unless stated otherwise. They are not intended for specification release and do not replace lot-specific certificate of analysis values.

When Melt Index Drops to 0.85 g/10 min, What Changes in Extrusion?

Because the 2.16 kg melt index is a low-shear capillary measurement, it underestimates the shear sensitivity of the polymer in a grooved-barrel extruder. The standard ASTM D1238 test imposes a nominal wall shear stress of approximately 1.94 × 10⁴ Pa in the melt index die; typical extrusion die shear stresses can exceed 10⁵ Pa. Consequently, the practical processing limit is not set by the 0.85 g/10 min value alone but by torque demand, die pressure, and melt temperature resulting from the molecular weight distribution. On a 90 mm single-screw extruder with a grooved feed section and 30:1 L/D ratio, the melt temperature should be controlled between 185 °C and 200 °C, with head and die zones set 5 K to 10 K higher than the barrel set points to avoid cold-surface melt fracture. At melt temperatures below 175 °C, start-up torque can approach or exceed 85% of drive rating on smaller extruders, and surface shark-skin defects are observed at the die exit. Above 240 °C, oxidation of the polyethylene backbone becomes measurable in residence times above 10 min, leading to viscosity reduction, odour, and a decrease in ESCR. The acceptable operating window for continuous output is therefore approximately ±15 K around the recommended melt temperature, which is wide enough for normal line control but too narrow for poorly tuned barrel heater bands or frequent line stops.

For thick-walled open-head drums with shot volumes above 20 L, parison programming is the primary control for uniform wall thickness. The lower melt index of the grade increases die swell relative to 1.5–2.0 g/10 min HDPE grades, so a diverging die gap in the range of 2 mm to 4 mm is usually required. Accumulator-head blow molding machines are preferred over continuous parison machines for parts above 30 L because the fast parison drop associated with the accumulator reduces sag during the interval between die exit and mould closure. Radial wall thinning of 20% to 35% at the top flash line and bottom pinch-off zone is typically programmed to compensate for trim losses and to prevent heavy pinch welds. Blow air pressure should be maintained at 0.5 MPa to 0.8 MPa for large parts, and mould temperature should be held at 10 °C to 30 °C to limit post-mould shrinkage. Semicrystalline HDPE with a nominal density of 0.955 g/cm³ exhibits mould shrinkage in the range of 1.5% to 2.5% after 24 h; thick walls cool slowly, and final dimensional stability may require 48 h of lay-flat storage before leak testing. Fast surface cooling without sufficient internal cooling time can create an outer frozen skin of 1–2 mm that temporarily masks shrinkage; such parts may pass initial gauging but show later diameter and top-load changes. Internal cooling air at −10 °C to 0 °C or a post-blow exhaust cycle can reduce cycle time by 10% to 20% in thick sections, but the exact reduction depends on mould steel thermal conductivity and cooling channel spacing.

When Open-Head Drums Require Environmental Stress Crack Resistance

Environmental stress crack resistance in DGDB-5004 NT 7 is assessed by ASTM D1693 condition B with 10% Igepal CO-630 at 50 °C. The supplier-published typical value exceeds 600 h, but this value is a comparative screening result, not a service-life prediction. Under actual service, moulded-in stress at pinch-off welds, sidewall flexure during handling, and contact with surfactants or organic liquids can reduce time to cracking. The density of 0.955 g/cm³ gives higher stiffness and chemical permeation resistance than HDPE grades below 0.950 g/cm³, while retaining sufficient slow-crack-growth resistance for non-pressure industrial containers. Grades with density above 0.958 g/cm³ may provide a 5% to 10% increase in top-load stiffness for equal wall thickness, but their ESCR often falls below the level required for stacked drums exposed to aqueous agricultural formulations. Conversely, grades below 0.950 g/cm³ can give higher ESCR but require thicker walls to meet the same column load, increasing part mass and cycle time. The selection logic for DGDB-5004 NT 7 is therefore based on a density window that avoids the brittle-failure side of the top-load/ESCR trade-off while still achieving transport-qualification top loads. For a 200 L open-head drum with a typical wall thickness of 2.0 mm to 2.5 mm, the material has been used in UN-certified packaging designs; however, final UN certification belongs to the moulded article and is not an inherent property of the resin. It should not be used for pressure-rated service such as PE100 pipe, where long-term hydrostatic strength must be demonstrated under ISO 9080 and ISO 12162.

For food-contact assessment in North America, the base olefin polymer may fall under FDA 21 CFR 177.1520 when the final article meets the extractive limitations for the intended food category and use temperature. Compliance is not automatically transferred from the resin supplier to the finished container; migration testing under the actual time-temperature profile is required for fatty foods, aqueous foods, and acidic foods when use exceeds 121 °C or involves long-term storage. The natural unpigmented grade does not contain cadmium, lead, hexavalent chromium, or mercury at levels above the maximum concentration values in EU RoHS Directive 2011/65/EU, but a final RoHS declaration must include any colour concentrate, UV stabilizer, or processing aid added by the converter. REACH registration for the polymer may be confirmed through the supplier safety data sheet; monomers and additives should be checked against the current candidate list for substances of very high concern. Because this product is supplied without a slip or antiblock additive package for film use, it is not interchangeable with HDPE film extrusion grades in thin-gauge applications.

Thermo-oxidative degradation is the main boundary in high-temperature extrusion

Thermo-oxidative degradation in HDPE begins with abstraction of hydrogen at allylic or tertiary positions, followed by hydroperoxide formation and chain scission. In extrusion, the rate of viscosity loss becomes measurable above 220 °C when oxygen is present. At the recommended melt temperature of 190 °C to 200 °C, residence time can be extended up to 12 min without significant degradation, but at 240 °C the same viscosity loss may occur in less than 6 min. The processing stabilizer package in DGDB-5004 NT 7 is designed for short extrusion heat histories, not for repeated reprocessing of regrind at high levels. Regrind addition should be limited to 20% to 30% of the total charge unless the converter verifies ESCR and tensile elongation retention on the finished part. Because the grade is unpigmented, degradation appears as yellowing that can be measured as an increase in yellowness index above 2.5 when viewed under a D65 illuminant; such yellowing indicates oxidation, not merely pigment variation. When the material must be stored before processing, it should be kept in a dry, covered area and protected from ultraviolet exposure. Black polyethylene storage bags are acceptable, but outdoor storage in translucent silos can lead to surface oxidation detectable as a decrease in ESCR before melt index changes.

Compared with HDPE blow molding grades having melt index values of 1.5–2.0 g/10 min, DGDB-5004 NT 7 gives higher parison melt strength at equal temperature but requires more extruder torque and produces higher die pressure. The lower melt index shifts the practical bottle weight range upward: it is not suitable for thin-wall dairy or personal-care bottles below 100 mL because long flow paths and rapid pin closing favour a more flowable grade. Compared with HDPE film grades based on the same density but higher melt index, the blow molding product lacks the cling, slip, and antiblock additives required for stable film bubble operation; it also has a different stabilization package. Compared with bimodal PE100 pipe resins, DGDB-5004 NT 7 is not assigned a minimum required strength rating of 10 MPa under ISO 12162, and it should not be used for internal pressure service. These differences are functional classifications rather than quality rankings. Converters selecting a replacement for another supplier’s 0.955 g/cm³, 0.85 g/10 min HDPE should compare the full shear viscosity curve, die swell, and ESCR rather than the two-point density and melt index values alone.

Drying of DGDB-5004 NT 7 is not normally required because high-density polyethylene has low equilibrium moisture uptake. However, if pellets are stored in unheated silos or transferred through cold lines into a warm plant, surface condensation can introduce moisture that appears as surging and surface porosity. A desiccant or hot-air hopper operated at 80 °C for 2 h is recommended only when condensation is observed or the ambient relative humidity exceeds 60%. Avoid processing the material with copper, manganese, or other transition-metal salts that catalyze hydroperoxide decomposition at elevated temperatures. If shutdown is planned, the screw and die should be purged with a higher-melt-index HDPE of 2.0 g/10 min or greater to reduce residual pressure on restart. Do not use a flame treatment intended for polypropylene on this polyethylene; high surface oxidation from excessive treatment can create low-molecular-weight species that reduce sealability and can impart taste or odour. End-of-life disposal should follow local plastics recycling streams for unpigmented HDPE; the absence of fillers allows mechanical recycling, but repeated thermal history shifts the oxidation induction time and must be evaluated before regrind reuse.