Why Does Sharkskin Persist in LLDPE Blown Film Even After Optimizing Screw Temperature Profiles?

Linear low-density polyethylene blown film exhibits a sharkskin defect as transverse ridges oriented perpendicular to the extrusion direction at the die exit when wall shear stress exceeds 0.14 MPa in narrow die gaps, although the precise threshold shifts with molecular weight distribution, die land length, and melt temperature. The addition of POLYBATCH™ XAMF-500-T Process Aid Concentrate based in LLDPE modifies the boundary condition at the die wall by depositing a low-surface-energy layer that converts stick-slip flow into stable plug flow, but this conditioning requires an induction period. On a production line fitted with a 75 mm grooved-feed extruder, 30:1 L/D, and a 1.8 mm die gap, die pressure commonly decreases by 8–20% within 20–60 min after switching to a compound containing 1.0 wt% of the concentrate; before the fluoropolymer layer is established, surface haze and sharkskin may persist and should not be interpreted as formulation failure.

Industry compliance for food-contact and agricultural film applications must address both the carrier resin and the final film. The LLDPE carrier falls under FDA 21 CFR 177.1520 for olefin polymers intended for food contact, while the finished monolayer or coextruded film must be evaluated under EU Regulation 10/2011 for overall migration and specific migration limits; the concentrate itself is subject to REACH registration and downstream communication obligations. The recommended addition ratio for blown film is 0.5–2.0 wt% of the concentrate, with a starting point of 1.0 wt% for converters running fractional-melt LLDPE with melt index below 1.0 g/10 min as measured by ISO 1133-1:2022. During scaled production, the concentrate is dry-blended with LLDPE granules via a gravimetric dosing unit at the main feed throat; pre-compounded pellets are avoided where layer ratio changes are required on site. Extrusion parameters include barrel zone temperature profile 160–200 °C, die temperature 200–215 °C, blow-up ratio 2.5:1–3.5:1, and frost line height 250–500 mm depending on bubble stability. Terminal product types include food contact freezer bags, bakery films, agricultural greenhouse films, construction vapour barriers, and collation shrink film; all exhibit reduced die lip build-up and lower gel incidence after the conditioning period.

Operational boundaries are material-specific. Addition above 2.5 wt% of the concentrate in thin-gauge film may reduce transparency because excess fluoropolymer at the die wall can generate low-amplitude haze bands, and inorganic antiblock packages containing silica or talc at levels above 2,000 ppm can compete for the die surface and delay conditioning by 30–60 min. The concentrate does not substitute for adequate melt filtration; a screen pack with 60/80/100 mesh layers remains necessary to remove degraded gel particles before the die.

When cast film lines run fractional-melt LLDPE above 250 m/min, edge tear and die lip build-up typically originate from high shear stress in the die land region, not from insufficient chill roll cooling. The use of LLDPE-based process aid concentrate at 0.3–1.5 wt% shifts the distortion regime to higher throughput by forming a stable coating on the die metal, but cast film has a shorter contact time between melt and die lip than blown film; therefore the level of fluoropolymer required at the die surface must be reached through higher initial addition and then stepped down. A typical step-down sequence starts at 1.2 wt% for the first 30 min, then 0.8 wt% for the following 30 min, then 0.5 wt% steady state, with die pressure and cast film haze monitored against ASTM D1003-21 and tensile properties against ASTM D882-18.

Compliance in cast film applications involves EU Regulation 10/2011 for hygiene and food contact laminates, FDA 21 CFR 177.1520 for the polyethylene carrier, and CONEG or EU Directive 94/62/EC for heavy metals in packaging. The downstream production process is a flat-die extrusion line with a 90–150 mm single-screw extruder, 30:1 L/D, barrier screw, coat-hanger die with adjustable lip gap 0.4–0.8 mm, vacuum box, air knife, and chill roll maintained at 15–30 °C. Terminal products include hygiene backsheet film, lamination film for coffee pouches, industrial stretch wrap, agricultural silage wrap, and surface protection film. Process aid use is particularly relevant in cast stretch film containing high levels of tackifier; metal stearates in tackifier masterbatches can deposit on the die and are displaced more readily when the PPA concentrate is present at 0.5 wt% or above.

Published data for this specific configuration is limited, but industrial practice indicates that the conditioning window may lengthen when chill roll release agents are used on the same line. Waxes and silicones from chill roll sprays can transfer to the die lip during edge trimming and interfere with fluoropolymer deposition; the mitigation is to purge with clean LLDPE for 20 min before reintroducing the additive.

When HDPE Pressure Pipe Lines Show Die Build-Up at Low Melt Temperatures

On pressure pipe lines operating at the low end of the melt temperature window to preserve molecular weight and slow crack growth resistance, the die exit can accumulate oxidized low-molecular-weight fractions from the outer layer of the melt stream. The concentrate based in LLDPE is compatible with HDPE pipe grades at 0.5–1.5 wt% addition, and the fluoropolymer layer reduces the adhesion of these oxidized fractions to the die land. On a pipe extrusion line using a 90 mm grooved-feed single-screw extruder with 33:1 L/D and a barrier screw, die pressure typically decreases by 6–15% over 45–90 min when the concentrate is introduced at 1.0 wt%, while melt temperature measured at the die adapter is held at 195–215 °C to avoid thermal degradation of the HDPE.

The regulatory framework for pressure pipe includes ISO 4427 for polyethylene piping systems, ISO 9080 for long-term hydrostatic strength extrapolation, ASTM D3350-21 for material classification, and NSF/ANSI 61 for potable water contact in North America; the carrier LLDPE must meet the same migration limits as the pipe compound when used in drinking water service. Downstream production involves dry-blending the concentrate with natural HDPE pipe resin at the main hopper, feeding through a screen pack of 60/80/100 mesh, forming the pipe through a spiral mandrel die, and calibrating under vacuum at water temperature 20–35 °C. Terminal products include PE100 pressure pipes for municipal water distribution, industrial slurry pipes, mining tailings pipes, telecom conduits, and corrugated drainage pipes. The process aid does not alter the hydrostatic design basis; long-term pressure ratings remain governed by the base HDPE resin and its ISO 9080 regression curve, not by the additive package.

Where low-smoke zero-halogen jacketing compounds contain more than 50 wt% ATH or MDH, the melt phase exhibits elevated viscosity and a pronounced tendency toward wall slip instability at conventional extrusion speeds. The LLDPE-based PPA concentrate is introduced at 0.8–2.0 wt% into the filled jacketing compound, with the higher end of this range reserved for formulations above 60 wt% mineral filler. On a 90 mm low-compression screw with 24:1 L/D and a 1.5:1 compression ratio, die pressure stabilization is typically observed within 30–90 min; melt temperature at the die is restricted to 140–170 °C because ATH begins endothermic dehydration near 180–200 °C and premature decomposition creates surface pitting and odour.

Regulatory compliance for wire and cable jacketing includes IEC 60502-1:2021 for power cable insulation and sheathing, BS 6724 for low-smoke halogen-free cables, RoHS Directive 2011/65/EU as amended, and REACH candidate list screening for SVHCs; the process aid concentrate must not introduce heavy metals or restricted flame retardants. The downstream process is single-screw extrusion with a vacuum vent, pressure-relief screen pack, and a crosshead die that applies the jacket over the insulated conductor at line speeds from 50–400 m/min depending on conductor cross-section. Terminal product types include low-smoke zero-halogen building wire jackets, photovoltaic cable sheathing, railway transit car wiring, and control cable jackets for marine and offshore installations. Dispersion quality is quantified by the filter pressure value test according to EN 13900-5:2005, with a maximum pressure rise criterion defined by the cable compound specification; the PPA concentrate reduces pressure fluctuations in the crosshead region but cannot compensate for severe filler agglomeration.

The concentrate is not a replacement for filler surface treatment. Halogen-free systems with high levels of zinc borate or magnesium hydroxide may delay fluoropolymer deposition onto the die wall; in these formulations, the initial addition should be held at 1.5 wt% for the first 45 min before stepping down to the steady-state level. Pre-drying is required at relative humidity above 60% because moisture uptake in LLDPE carrier can generate surface voids in the jacket wall.

Die Drool in Extrusion Blow Moulding with Post-Consumer Recyclate Streams

In extrusion blow moulding, post-consumer recyclate introduces polar contaminants, printing ink residues, and metal stearate soaps that accumulate on the die face as drool and then transfer to the parison surface. A LLDPE-based process aid concentrate at 0.5–1.2 wt% reduces the adhesion of these residues by coating the die lip and promoting slip at the metal–melt boundary. The addition ratio is raised to 1.2 wt% when the recyclate content exceeds 25 wt% or when the melt index of the base HDPE falls below 0.4 g/10 min at 190 °C/2.16 kg per ISO 1133-1:2022. Production-scale observations on shuttle blow moulding machines with 60–120 mm extruders and parison die diameters from 40–200 mm show that die face cleaning intervals can be extended from 4–8 h to 16–24 h once the fluoropolymer layer is conditioned, but this improvement is lost if the die is aggressively cleaned with abrasive pads during the conditioning period.

Compliance standards include FDA 21 CFR 177.1520 for olefin polymer articles in food contact and EU Regulation 10/2011 for food containers; where the bottle contains post-consumer recyclate, EU Regulation 2022/1616 governs the suitability of the recycling process and the use of recycled material in food contact. The downstream process is extrusion blow moulding with a continuous or accumulator head, parison programmer, and mould clamp force typically between 100–500 kN for bottles up to 5 L; melt temperature is controlled between 180–210 °C to balance parison sag against surface finish. Terminal products include HDPE detergent bottles, household chemical containers, dairy bottles, personal care bottles, and industrial jerrycans. The additive does not increase melt strength and does not prevent parison sag in high-melt-index resins; sag resistance must be controlled by selecting a suitable blow moulding grade and parison programming.

For extrusion coating lines applying low-density polyethylene to paperboard or aluminium foil, the narrow air gap and high draw ratio amplify any melt flow disturbance originating at the die lip. The LLDPE-based process aid concentrate is let down at 0.5–1.0 wt% in the coating resin; because the melt curtain is drawn down to 8–25 µm thickness, even minor die face oxidation can cause web breaks or edge pinning instability. In industrial operations using a 120 mm extruder with 30:1 L/D and a coat-hanger die with 0.4–0.8 mm lip gap, the addition of PPA concentrate stabilizes the melt curtain and reduces edge neck-in by 2–5 mm under constant tension, but the effect is less pronounced when corona treatment is not applied before winding.

The regulatory framework for food-contact extrusion coating includes FDA 21 CFR 176.170 for components of paper and paperboard in contact with aqueous and fatty foods, FDA 21 CFR 175.300 for resinous and polymeric coatings, and EU Framework Regulation 1935/2004 with overall migration testing under EN 1186-1:2002. The downstream production process is an extrusion coating and lamination line with a chilled polishing roll maintained at 10–20 °C, a pressure roller system with nip loads from 50–150 N/mm, and an inline corona treater operating above 42 mN/m surface energy. Terminal product types include liquid carton board for milk and juice, sachet and stick pack laminations, food service cup stock, pharmaceutical blister lidding foil, and aseptic packaging. The fluoropolymer can migrate to the coating surface and reduce wetting tension; inline corona treatment of the coated web to 44–48 mN/m is therefore part of standard operating practice before lamination or printing.