On blown-film lines converting LLDPE-rich formulations into agricultural mulch and silage-cover film, POLYBATCH™ Dark Grey LL80362/4 is metered into the main feed throat at a mass let-down ratio of 2–5 wt%, with the upper end of the range reserved for films below 15 µm and the lower end used when high calcium carbonate filler levels already contribute opacity. The concentrate carrier is LLDPE, which limits melt-viscosity mismatch when the base resin is a high-molecular-weight film grade with a melt mass-flow rate of 0.9–1.2 g/10 min under ISO 1133-1:2022 at 190 °C and 2.16 kg. A barrier screw with a length-to-diameter ratio of 25:1–30:1 and a feed-throat temperature of 35–50 °C is recommended, because premature softening of the concentrate at the feed opening can create intermittent feed slugs and visible grey bands in the layflat. A graduated screen pack of 60/80/100 mesh retains undispersed pigment agglomerates larger than 100 µm, while a typical barrel profile from feed to die is 160 °C, 180 °C, 200 °C, 210 °C with adapter and die at 210–220 °C. The melt temperature measured at the die exit is kept below 240 °C to avoid oxidative chain scission in the LLDPE carrier, and the bubble is run at a blow-up ratio of 2.2:1–3.0:1 with a frost-line height of 4–8 die diameters. Raising the frost line beyond this range with dark grey formulations increases surface migration of low-molecular-weight pigment carrier fractions and can raise blocking forces on wound film. Tensile properties of 12–20 µm mulch film are evaluated under ASTM D882-18, and light transmission is checked by ASTM D1003-21; the concentrate is expected to reduce transmission progressively with addition level, but published data for this exact product configuration is limited, so end-use validation under ISO 4892-2 xenon-arc weathering is required before UV-screening performance is guaranteed. At 2 wt% addition the film may retain slight translucency in the thinnest gauges, while at 5 wt% the surface approaches full visual opacity but the bubble can become more sensitive to tower draughts and gauge variation. The concentrate should be pre-dried at 60–70 °C for 2–4 h when ambient storage humidity exceeds 60% RH; moisture carried into the melt through high-surface-area pigment particles otherwise forms fisheyes and pinholes that are difficult to remove without shutting down the tower.

What Limits Concentrate Addition Rate in Geomembrane Sheet Extrusion?

In flat-die sheet extrusion for LLDPE-based geomembranes with thicknesses from 0.75 mm to 2.0 mm, the concentrate is usually compounded at 3–6 wt% into a medium-density or high-density polyethylene-rich matrix; the LLDPE carrier is miscible enough in polyolefin blends that it does not form visible weld-line boundaries, but its lower melt viscosity reduces the bulk melt strength of thick sheet at higher addition rates. A 90 mm single-screw extruder with a 30:1 L/D and a coat-hanger die opened to 1.0–1.5 mm is operated with a melt temperature of 195–230 °C, while the first polishing roll is held at 60–80 °C and the second at 40–55 °C; higher roll temperatures force low-molecular-weight pigment carrier fractions to deposit on the roll surface as plate-out, which subsequently transfers back to the sheet as dull grey streaks. Concentrate addition above 6 wt% increases the risk of edge waviness and transverse gauge variation across a 4 m web, and the increased pigment loading shifts the sheet coefficient of linear thermal expansion, making seam-weld alignment more sensitive to ambient temperature changes; this property is verified under ASTM D696-16 if the geomembrane is installed with welded seams. Dark grey sheet also absorbs more incident radiation during the polishing-stack cooling step, so the temperature difference between the sheet core and the sheet surface can widen if the polishing-roll water flow is not adjusted, resulting in residual stress that appears as curl at the cut edges. Oxidative induction time is tested under ISO 11357-6:2021 at 200 °C in oxygen, and the dark grey concentrate must not reduce the OIT of the formulated antioxidant package below project-specific minimums; high-surface-area carbon black can adsorb a fraction of the phenolic stabilizer during melt mixing, an effect that becomes measurable at addition levels above 5 wt% unless the base resin contains additional stabilizer. The concentrate should be pre-dried at 60 °C for 2 h if opened bags are stored at ambient humidity above 70% RH, and it must not be combined with amine-based antistatic masterbatches because such additives can react on the pigment surface and produce low-level organoleptic contaminants in confined installation environments.

Where gauge uniformity below 20 µm governs the conversion of refuse sacks and heavy-duty industrial liners, the dark grey concentrate is usually introduced into LLDPE by direct dosing at 2–4 wt%, with the carrier resin selected to match the melt mass-flow rate of the base polymer within 0.3 g/10 min under ISO 1133-1:2022. On a high-output blown-film line with an extruder L/D of 24:1–30:1 and a die diameter of 150–300 mm, the head pressure is monitored continuously; a pressure increase greater than 15% over the start-up value usually indicates pigment agglomerate build-up on the 60/80 mesh screen pack, which is then replaced every 6–12 h during prolonged runs. The blend is processed at 190–220 °C for film thicknesses of 15–25 µm, and dart-impact resistance is evaluated under ASTM D1709-22 method A or B depending on the expected service load; because dark pigment particles act as stress concentrators in thin sections, the let-down ratio is deliberately reduced toward 2 wt% when low-temperature impact resistance at -20 °C is required. A grooved-feed extruder section with forced cooling prevents pre-melting and slug feeding of the concentrate, which would otherwise create light grey streaks that are visible against a backlit inspection light table. The thickness profile is scanned by an oscillating capacitive gauge, and deviations outside ±2 µm are corrected by adjusting the internal bubble cooling airflow rather than altering the let-down ratio, because pigment distribution responds more slowly to screw-speed changes than the air ring. Compliance for refuse-sack waste-stream applications is generally assessed against REACH Article 33 and EU Directive 94/62/EC; if the dark grey concentrate is supplied with food-contact declarations, migration testing of the finished film under EU Regulation 10/2011 and FDA 21 CFR 178.3290 remains the responsibility of the converter because pigment purity and addition-rate limits are not universally food-grade across all packaging configurations.

When Cast Film Line Speeds Exceed 120 m/min for Stretch Hoods

For cast pallet-hood and silage-wrap constructions where line speeds above 120 m/min are typical, the dark grey concentrate is metered at 3–5 wt% into an LLDPE base with a melt mass-flow rate of 2.0–3.5 g/10 min under ISO 1133-1:2022. The cast-film process gives the concentrate a shorter thermal residence time than blown film, which can reduce pigment heat history, but it places a premium on rapid pigment wetting because the melt travels from die exit to chill roll in a fraction of a second. A slot die with a 0.4–0.6 mm die gap is held at 220–240 °C, and the melt curtain is pinned to a polished chill roll at 20–30 °C using a vacuum box or electrostatic edge-pinning system. Surface roughness from poorly dispersed pigment agglomerates larger than 50 µm can disturb the pinning edge and create periodic gauge bands across the web; these bands are not acceptable in stretch hoods because they convert into premature tearing under elongation. The resulting cast film is evaluated for elongation at break under ASTM D882-18; typical dark grey stretch-hood films require machine-direction elongation above 500% and transverse-direction elongation above 700%, although the actual values are set by the converter's base resin and not by the concentrate alone. Because the dark grey film absorbs more infrared radiation during the draw, the chill-roll surface temperature is controlled within ±2 °C; a higher roll temperature can cause blocking, while a lower temperature can increase transverse crystallinity gradients and reduce tear resistance. A gravimetric dosing unit with 0.1 wt% accuracy is used to avoid minor let-down fluctuations that would appear as visible dark streaks at these line speeds. Moisture pickup above 0.05% by weight in the pigment concentrate is sufficient to create moisture marks on the cast web, so closed hopper storage under nitrogen purge at 0.1–0.3 bar is applied in high-humidity production halls.

Screw Back Pressure in Injection Moulding Sets the Homogeneity Limit

In polyethylene injection-moulding grades used for storage bins, agricultural clips, and material-handling components, POLYBATCH™ Dark Grey LL80362/4 is added at 2–4 wt%, and the converter confirms that the LLDPE carrier does not exceed 10 wt% of the total polymer matrix to avoid softening the part and reducing heat deflection temperature under ISO 75-2:2022 method A at 1.8 MPa. A general-purpose injection screw with a 20:1 L/D and a compression ratio of 2.5:1–3.0:1 is sufficient, but the screw back pressure is set to 0.5–1.5 MPa to improve dispersion homogeneity without generating excessive shear heat. Melt temperature at the nozzle is held at 200–230 °C for polyethylene-based parts, and the mould surface temperature is set at 20–40 °C; dark pigment systems respond poorly to excessively high injection speeds because the melt front can orient pigment particles along the filling path, leaving flow lines that are visible on the part surface. Weld-line regions such as hinge intersections or boss junctions are particularly sensitive because pigment particles accumulate at the melt front and can produce a lighter grey weld line that is structurally sound but aesthetically rejected. Tensile property verification on injected plaques is performed under ISO 527-2:2022 at 50 mm/min; if the part is intended for outdoor agricultural service, accelerated weathering under ISO 4892-2 with a xenon lamp and a black-standard temperature of 65 °C is used to quantify colour stability and surface chalking. The mould must not be pre-treated with silicone sprays because such sprays can alter the surface gloss of dark grey parts and create a non-uniform release profile that mimics pigment streaking. When the concentrate is stored in partially opened bags above 60% RH, pre-drying at 60–65 °C for 2 h is applied before injection to prevent moisture-induced silver streaks in thick sections; this is especially important when the component contains a flat closure face that is inspected under direct light.

In extrusion coating of dark grey tie layers or surface layers onto paper, board, or aluminium foil in industrial packaging, the concentrate is introduced at 4–7 wt% into an LLDPE or LDPE coating grade with a melt mass-flow rate of 7–15 g/10 min under ISO 1133-1:2022. A 90–120 mm single-screw extruder with a barrier screw feeds a slot die with a die gap of 0.8–1.2 mm, mounted above a chill roll and pressure roll arrangement. Melt temperature at the die is 250–280 °C to promote substrate wetting, but the concentrate is not held at this temperature for more than 5 min because the high pigment surface area accelerates thermal-oxidative degradation of the LLDPE carrier, leading to odour and pinholes in the thin coating. Coating weight is typically 15–30 g/m², and the pigment dispersion must be sufficiently fine that no visible specks above 25 µm appear on the coated surface; this is checked by light microscopy at 50× magnification under a halogen backlight. Adhesion to the substrate is tested by tape peel per ASTM D3359-23 method B, and pigment agglomerates that protrude through the thin coating layer can reduce adhesion ratings from 5B to 3B or lower. The dark grey colour in thin coating layers is sensitive to dilution; below 4 wt% there may be visible variation in optical density when the substrate colour is inhomogeneous, and above 7 wt% the melt curtain can lose stability at high line speeds because the particulate phase interferes with the extensional flow field. The coated surface temperature on the chill roll is maintained at 15–20 °C to prevent blocking, and the take-off speed is limited by the cooling capacity of the chill-roll diameter and the air-gap length rather than by a single published speed limit. If the concentrate is used in direct food-contact surface layers, the finished article must be tested for overall migration under EU Regulation 10/2011 and the pigment package must meet the purity requirements of FDA 21 CFR 178.3290; for non-food industrial packaging, REACH SVHC screening and RoHS Directive 2011/65/EU Annex II restrictions are the applicable baseline.

Because outdoor micro-irrigation tube and drip-tape formulations require a balance between UV opacity and dimensional stability, the concentrate is typically metered at 2–5 wt% into LLDPE-rich irrigation compounds, and the LLDPE carrier is not used as the sole carrier in high-pressure PE80 or PE100 pipe formulations unless the converter has verified long-term hydrostatic strength under ISO 9080:2022. A single-screw extruder with a 25:1–30:1 L/D and a pipe die with a land length ratio of 10:1–15:1 is operated at melt temperatures of 190–220 °C, with vacuum sizing controlled at -0.2 to -0.6 bar to maintain outer diameter tolerance. The concentrate is pre-dried at 60–70 °C for 2–3 h if exposed to ambient humidity above 60% RH, and the extruder head pressure is monitored so that an increase above 250 bar triggers screen-pack replacement for pigment agglomerates. Slow crack growth resistance of the final pipe is assessed by the ISO 13479-1:2022 notched-pipe test, and because dark grey concentrates contain carbon black, the dispersion quality directly influences brittle failure initiation; the let-down ratio is therefore limited to 5 wt% and the screw back pressure is set to 0.8–1.5 MPa to prevent undispersed pigment clusters larger than 50 µm. In agricultural drip tapes, the dark grey colour reduces visible algae growth when the tape is exposed to sunlight, but the addition must not reduce burst pressure; hydrostatic burst testing is conducted under ISO 1167-1:2022 at 20 °C and 60 °C depending on the application class. Published data for this specific grade in drip-tape extrusion is limited; start-up trials should begin at 2 wt% and increase only after extrusion pressure, dispersion screening, and burst-strength data are collected.