When Coextruded Fuel Tank Shells Use DMDY1158 as the Structural HDPE Layer

In automotive fuel tank coextrusion, Sinopec Qilu DMDY1158 is specified as the structural polyethylene phase because the grade combines a nominal density of 0.958 g/cm³ under ISO 1183-1:2019 with a melt flow index of approximately 0.6 g/10 min under ISO 1133-1:2022, a combination that provides sufficient parison melt strength for tank lengths above 1.2 m without excessive sink marks or pinch-off thinning. Published technical data for this grade also cite an environmental stress crack resistance F50 value exceeding 600 h under ASTM D1693-15 Condition B at 50 °C, which is relevant because fuel tank shells are exposed to long-term cyclic hydrocarbon contact, road-borne flexural stress, and low-temperature impact at -40 °C under GB 18296-2019 thermal shock and low-temperature drop procedures. DMDY1158 is not used as a monolayer fuel barrier; it functions as the structural outer, regrind, and inner layers in a coextruded wall that incorporates an EVOH hydrocarbon barrier. The allowable coextrusion processing envelope is narrower than monolayer industrial blow moulding because EVOH begins to degrade exothermically above 235 °C, while tie-layer adhesion to EVOH deteriorates if the interfacial tie layer drops below approximately 2 μm, producing localised delamination that fails permeation testing under UN/ECE R34 and GB 18296-2019.

Industry compliance for automotive fuel tank applications includes mechanical integrity tests under UN/ECE R34 for fire resistance, impact resistance, and fuel leakage after crash-simulated loading, as well as GB 18296-2019 for fuel tank safety performance in the Chinese domestic market. Hydrocarbon permeation is evaluated by gravimetric or sniffing methods specific to evaporative emission certification; converters should verify whether the final vehicle platform falls under EPA 40 CFR 86.1813 evaporative emission families or equivalent national limits. The formulation addition ratio in a six-layer tank wall typically places DMDY1158 at 88–92 wt% of the finished wall, with EVOH at 2–3 wt%, maleic anhydride grafted polyethylene tie resin at 4–6 wt% total across both tie layers, and carbon black masterbatch at 2.0–2.5 wt% of the outer layer only for UV shielding. Regrind generated from flash and rejected tanks may constitute 20–40 wt% of the regrind layer, but each 10 wt% incremental increase in regrind content should be revalidated against ASTM D1693-15 because repeated thermal history reduces ESCR and may alter the pinch-off weld toughness.

The downstream production process utilises a coextrusion blow moulding line with a main HDPE extruder of 90 mm screw diameter and 24D L/D, a 35 mm EVOH extruder, and two 25 mm tie-layer extruders feeding a six-layer die head maintained at 220–235 °C. Melt temperature for DMDY1158 is controlled at 205–220 °C, while EVOH is maintained between 210 °C and 230 °C to avoid gel formation at the barrier-tie interface. A die gap programmer opens the die gap from 2.0 mm to 3.5 mm during parison drop to compensate for sag-induced thinning at shot lengths above 1.5 m. Blow air pressure is held between 0.6 MPa and 0.9 MPa, mould coolant temperature is maintained at 12–15 °C, and clamp force for passenger car fuel tank moulds is typically 200–400 tonnes. Cycle time ranges from 90 s for small automotive tanks to 180 s for large commercial vehicle tanks. On production-scale lines, the primary observed failure mode is not throughput loss but wall-thickness variation exceeding ±0.3 mm across the chime and pinch-off regions, which can reduce local EVOH thickness below the permeation design target and cause downstream evaporative emission test failure. Terminal finished product types include gasoline and diesel fuel tanks for passenger cars, SUV and light commercial vehicle tanks, off-road equipment tanks, and blended-fuel tank shells where the barrier layer is specified for oxygenated fuel contact.

What Constrains Parison Stability in 210 L Tight-Head Drum Extrusion Blow Moulding?

A 210 L L-ring tight-head drum produced from DMDY1158 typically has a finished wall mass of 8.5–10.5 kg and a body wall thickness distribution of 1.8–3.2 mm, with the chime and bottom corner areas controlled to 3.0–3.5 mm to meet stacking and drop-test requirements. DMDY1158 is selected in this application because its high melt strength, derived from its nominal 0.6 g/10 min melt flow index under ISO 1133-1:2022, enables the 9–12 kg parison shot to be dropped without sag-induced pinching at the accumulator head. The primary process limitation on larger drums is not extruder delivery but parison sag at shot weights above 10 kg; when parison sag exceeds approximately 5–8% of initial diameter before mould close, wall thinning at the fill point or pinched flash line can fall below the minimum required for the UN drop test. Converters using grooved-feed single-screw extruders with 120 mm screw diameter and 25D L/D report that melt temperature must be kept between 190 °C and 215 °C to avoid both unmelted gels at the die head and excessive sag at the upper temperature limit.

Compliance standards for this application include UN Model Regulations 6.1.5 for dangerous goods packagings, ADR 6.1.5 and IMDG Code provisions for intermediate bulk and drum packagings, ISO 16101:2004 for compatibility testing of polyethylene packaging with liquid chemicals, and GB/T 325.1 for steel-reinforced or plastic drum dimensional interfaces where applicable. The formulation addition ratio in standard industrial drum production is 97.0–98.0 wt% DMDY1158 neat resin with 2.0–3.0 wt% carbon black masterbatch using an LLDPE carrier; processing aids are limited to 0.5–1.0 wt% only when surface melt fracture appears at high screw speeds. Clean in-house regrind is added up to 30 wt%, but regrind above this concentration requires revalidation of ESCR under ASTM D1693-15 because each pass through the extruder increases oxidative history and lowers the F50 value. No amine-based slip or antistatic concentrates are used in UN-certified drum formulations unless specifically required and revalidated, because polar additive packages can reduce pin-to-pin ESCR and alter weld-line strength in the chime area.

The downstream production process for 210 L tight-head drums uses an accumulator blow moulding machine with shot capacity of 12 kg, a 120 mm grooved-feed single-screw extruder at 24D L/D, and a clamp unit of 1,000–1,500 kN. Barrel zone temperatures from feed to metering are set at 180 °C, 195 °C, 205 °C, and 215 °C, while the accumulator head is held at 205–215 °C. Blow air pressure is 0.7–0.9 MPa, mould coolant inlet temperature is 12–15 °C, and total cycle time for a single-station line is 150–240 s. Pellets stored at relative humidity above 60% may carry surface moisture, causing intermittent surging, pinholes, and melt fracture at the die lip; in such conditions, dehumidified-air hopper drying at 70–80 °C for 2–4 h is applied as an operational boundary. Terminal finished product types include 50 L, 120 L, 200 L, 210 L, and 220 L tight-head and open-head drums for dangerous and non-dangerous liquid chemicals, as well as 20 L, 30 L, and 60 L UN-certified jerricans.

Pesticide Container Shelf-Life Compliance Under GB 3796-2018 and UN 1H1 Stress-Crack Protocols

Agrochemical packaging made from DMDY1158 is driven by the need to contain emulsifiable concentrates, suspension concentrates, and solvent-based formulations for multi-season storage in outdoor depots. Many pesticide formulations contain aromatic hydrocarbon solvents, cyclohexanone, or dimethylformamide fractions, which accelerate stress cracking in low-density and medium-density polyethylene grades. DMDY1158 is therefore evaluated against ASTM D1693-15 Condition B and against chemical compatibility tests under ISO 16101:2004 using the actual formulation or a standard test liquid specified by the packager. Industry compliance standards in China include GB 3796-2018 for pesticide packing, GB 191-2008 for packaging marking for dangerous goods, and GB/T 4857.5-1992 for drop testing of filled containers. Export pesticide containers fall under UN Model Regulations 6.1.5 for 1H1 drum-type plastic packagings, with the packing group assigned by the oral or dermal toxicity class of the active ingredient and the solvent flash point. The maximum nominal capacity for this application is generally 20 L, above which paint or solvent drum standards usually supersede pesticide container standards.

The formulation addition ratio for long-life pesticide containers uses DMDY1158 at 94.0–97.0 wt% with a UV-stabiliser masterbatch at 3.0–5.0 wt% and carbon black masterbatch at 1.0–2.0 wt%. The UV-stabiliser masterbatch is normally a hindered amine light stabiliser or benzotriazole-based system compounded in the same HDPE carrier to avoid viscosity mismatch. Titanium dioxide or coloured masterbatch is limited to 1.0–2.0 wt% for label contrast and is not substituted for UV-stabilised grades when containers are stored in unshaded distribution yards. Highly aggressive solvent systems with aromatic hydrocarbon content above approximately 30% may require post-moulding fluorination of the internal surface rather than a change in resin formulation. Fluorination treatment typically uses a 0.2–0.5% fluorine-in-nitrogen gas mixture for 10–60 s to create a low-permeation fluorinated surface layer, but the treated container must be revalidated for wall discoloration and surface abrasion resistance under GB 3796-2018.

The downstream production process for 1 L to 20 L pesticide bottles and jerricans is single-station or dual-station extrusion blow moulding with a 65 mm 24D single-screw extruder, melt temperature controlled at 200–215 °C, and accumulator or continuous parison heads depending on container weight. Container weights range from 90 g for a 1 L bottle to 1.2 kg for a 20 L jerrican. Blow air pressure is 0.6–0.8 MPa, mould coolant temperature is 10–14 °C, and cycle time is 20–50 s for small bottles and 90–150 s for jerricans. The critical process control point is the pinch-off weld at the base and handle regions, because incomplete melt fusion creates a stress-concentrated channel through which aromatic solvents can penetrate the wall. Terminal finished product types include 500 mL, 1 L, 5 L, 10 L, and 20 L narrow-neck HDPE bottles and jerricans for insecticides, herbicides, fungicides, and plant growth regulators, including transparent dosing chambers where the DMDY1158 inner layer is coextruded with a thin clear polyethylene or polypropylene display strip.

In potable water and food-contact applications, DMDY1158 is evaluated less through tensile performance than through migration-limit compliance and organoleptic neutrality under long-term wet contact. The applicable compliance standards include FDA 21 CFR 177.1520 for olefin polymers, EU No 10/2011 and amendments for plastic food-contact materials, GB 4806.7-2023 for food-contact plastic articles in the Chinese domestic market, and NSF/ANSI/CAN 61 for potable water system components where the container is installed in drinking water distribution equipment. For potable water storage tanks, overall migration must not exceed 10 mg/dm² under EU No 10/2011, and sensory analysis must show no detectable taint under GB 4806.7-2023 Annex C. The formulation addition ratio is typically 97.0–98.0 wt% DMDY1158 with 2.0–3.0 wt% titanium dioxide white masterbatch or 0.5–1.0 wt% phthalocyanine blue masterbatch. Post-consumer regrind is not used in food-contact layers unless covered by an explicit compliance statement; in-house clean edge trim and flash from the same food-contact lot are permitted under EU No 10/2011 but must be revalidated for migration after each regrind loop.

The downstream production process for potable water containers differs from industrial drum production in that the mould cooling system must avoid rust-inhibiting water additives that can leave surface residues, and the extruder must be purged thoroughly after processing non-food grades. A typical 20 L returnable HDPE water container is blow moulded on a 55 mm 24D single-screw extruder at melt temperature 190–205 °C, mould coolant temperature 12 °C, and blow air pressure 0.6–0.7 MPa. Cycle time is 35–60 s. For larger potable water tanks up to 200 L, accumulator blow moulding with a 90 mm 24D extruder and melt temperature 195–210 °C is used. Terminal finished product types include 18.9 L returnable water containers, 20 L potable water jerricans, 50–200 L drinking water storage tanks for camping vehicles and off-grid systems, and food-grade intermediate bulk container liners where the DMDY1158 shell is specified only as the outer structural layer and a separate food-contact liner governs migration compliance.

Marine flotation modules and double-wall traffic barriers expose DMDY1158 to long-term ultraviolet radiation, cyclic flexural loading, and continuous water immersion rather than to chemical stress cracking. The compliance standards in this low-regulatory-burden segment are mainly weathering and mechanical codes rather than food or dangerous-goods regulations. Relevant test designations include ISO 4892-3:2016 for UV fluorescent weathering, ASTM G154-16 for accelerated UV exposure, ISO 527-2:2012 for tensile yield retention after weathering, and ISO 899-2:2003 for tensile creep in plastics under load. The formulation addition ratio must be biased toward UV protection: DMDY1158 is blended with 4.0 wt% carbon black masterbatch of particle size 20–50 nm to provide UV stabilisation, and 0.5–1.0 wt% processing antioxidant masterbatch may be added when extended outdoor service beyond 10 years is specified. Published long-term creep-rupture data for DMDY1158 under continuous brackish water immersion and cyclic flexural loading are limited; validation through ASTM D2990-17 and ISO 899-2:2003 is required before specifying the grade for load-bearing marine structures.

The downstream production process for large flotation shells uses accumulator blow moulding with shot capacity up to 30 kg, a 120 mm 24D single-screw extruder, melt temperature 195–215 °C, blow air pressure 0.8 MPa, and mould coolant temperature 10 °C to reduce cycle time. The primary processing failure mode is insufficient pinch-off weld strength at the outer rim, where multiple parison layers are compressed; weld-line thickness below approximately 2.5 mm can crack under cyclic wave loading. Terminal finished product types include dock floats, aquaculture cage collars, double-wall traffic barriers, cable pull boxes, and insulated flotation chambers for floating solar platforms.