When a 15 µm Hygiene Backsheet Requires Opacity Retention After Machine-Direction Stretching

Blown-film converters running white outer layers on coextruded hygiene backsheet typically dose LyondellBasell POLYBATCH™ White LL8350 AP at 6–10 wt% in the two skin layers. The LLDPE carrier melts in the same 190–210°C melt-temperature window as metallocene LLDPE and conventional LLDPE film resins; this prevents the film-haze and die-lip instability that can appear when a LDPE-carrier white concentrate is blended into an mLLDPE-rich formulation at ratios above 5 wt%. On a 65 mm 24:1 L/D barrier screw fitted with a Maddock shear-mixing section of 2–3 flights, the masterbatch is metered gravimetrically at the main hopper. The base resin and masterbatch pass through a 100/150/100 mesh screen pack or a 120-mesh candle filter; as the screens load with gel and TiO₂ agglomerates, melt pressure at the breaker plate rises from 180–260 bar to 300–350 bar over an 8–12 h run, after which the screen pack is changed. Barrel set points are held at 175°C in the feed zone, 195–205°C in the compression zone, and 205–215°C in the metering zone; the die is set at 205–215°C. The blow-up ratio is kept at 2.0:1–2.6:1, with a frost line height of 500–800 mm from the die face, because higher blow-up ratios increase transverse orientation at the bubble and can expose pigment agglomerates as pinholes in a 15–18 µm backsheet. The film is produced with a total luminous transmittance below 15% under ASTM D1003-13 and a CIE whiteness index above 70 under ISO 11475:2017.

The critical processing boundary is the dart-drop penalty. In a three-layer structure with a skin-to-core ratio of 20/60/20, raising masterbatch from 6 wt% to 10 wt% in the outer skin increases the total TiO₂ solids in the skin to approximately 3–5 wt% if the concentrate is specified at 50% active TiO₂; the dart-drop value measured under ASTM D1709-16a Method A typically tracks below the control film because highly filled white skins reduce crack-propagation resistance in the machine-direction stretch zone. Converters should validate the finished backsheet against the end-use dart specification; published data for this specific grade in a 15 µm hygiene laminate is limited. The film is tested for modulus and elongation at break under ASTM D882-18, for Elmendorf tear under ASTM D1922-15, and for surface roughness under ISO 8791-4 if the backsheet is later printed or adhesively laminated. Pre-drying of LL8350 AP is not normally required because the LLDPE carrier is non-hygroscopic; if the pellets are stored in an unheated warehouse with high relative humidity, condensation on the pellet surface can generate visible micro-voids. In that case, the material is dried at 60°C for not more than 4 h, because higher temperatures or longer residence times can soften the pellets and cause bridging in the hopper.

In cast-film operations running 800–1,100 m/min, LL8350 AP is metered at 3–6 wt% into the white skin of a three-layer silage wrap or temporary greenhouse cover. The masterbatch is fed into a 75 mm 30:1 L/D skin-layer extruder; because the heat transfer in a cast-film barrel is less forgiving than blown film, the feed-zone set point is kept 10–15°C above the resin's softening range, and the remaining zones are raised to 210–230°C. The pigment-loaded melt is filtered through a 120-mesh disk pack; if pressure at the screen changer increases more than 25–35 bar within a shift, the TiO₂ dispersion or screen-pack loading is considered out of control. The white skin is combined with a metallocene LLDPE core and a second white or UV-stabilized skin in a 3-layer feedblock; the skin layers are held at 10–15% of total thickness to avoid reducing the film's tear propagation resistance. The film is cast onto a chill roll at 18–25°C, with an air gap of 55–90 mm; excessive air gap reduces the white skin's adhesion to the core and creates machine-direction streakiness when the TiO₂ particles are reoriented.

The main processing constraint in silage wrap is the tackified cling layer. LL8350 AP should not be dosed into the cling layer because the particulate TiO₂ migrates to the surface and disrupts the tackifier film, changing the coefficient of friction measured under ASTM D1894-11 and reducing the self-adhesion needed for bale sealing. The masterbatch is kept entirely in the white outer skin; if a fully opaque white film without a separate cling layer is required, the tackifier is added to the outer skin through a separate cling-agent masterbatch after the white concentrate has been dispersed, not before. For a 200 µm white greenhouse film, the letdown is adjusted upward to 8–10 wt% in the two outer layers because UV screening and radiant-energy reflection are required over multiple seasons. The resulting film is tested for tensile properties under ASTM D882-18, dart-drop under ASTM D1709-16a, and accelerated weathering under ASTM G155-13 cycle 1; the TiO₂ surface absorbs UV and protects the inner LLDPE phase from chain scission. In high-speed cast-film operations, the use of an additive processing aid at 0.02–0.05 wt% may be required if the white concentrate increases die-lip buildup; the processing aid is added separately, not blended into the masterbatch.

What Limits TiO₂ Dispersion in Rotational Molding Dry Blends?

Rotational molders dry-blend LL8350 AP with 35-mesh or 60-mesh LLDPE micropowder at 2–6 wt%. The concentrate's pellet geometry matters more in rotomolding than in film extrusion because the dry blend is tumbled for 20–40 min in a ribbon or drum blender, then transferred with pneumatic conveying. If the masterbatch pellets are too large relative to the powder, vibration during transport segregates the pigment and produces a part with visible white streaks, uneven wall thickness, and variable opacity. The particle-size distribution is checked by dry sieve analysis under ASTM D1921-18; the target is less than 2% retained on a 60-mesh sieve when the powder is 35-mesh, because oversized pigment carriers remain isolated during early powder coalescence. If the concentrate is available only as standard film-cut pellets, the rotomolder grinds the masterbatch through a 20,000 rpm high-speed mill and re-sieves before blending; published data for this specific ground configuration is limited, so the powder blend is validated by molding a flat plaque and examining the distribution.

The oven cycle is set at 260–290°C for LLDPE-based charges, with a peak internal air temperature of 200–220°C and a cook time of 25–35 min at 3–5 mm nominal wall. TiO₂ increases the melt viscosity at zero shear, so a formulation that molds cleanly at 4% may show trapped bubbles, pinholes, and poor inside-surface gloss at 8% because the denser, more viscous melt cannot release air through the biaxial rotation path. The mold rotation ratio is held at 4:1 primary to secondary; if the arm ratio is changed to improve wall uniformity, the elevated viscosity from the white concentrate can shift the internal air pocket and create a knock-out rib or a thin bottom corner. The molded part is tested for tensile elongation under ASTM D638-14, notched Izod impact under ASTM D256-10, and low-temperature drop impact under ISO 6603-2:2000. White rotational-molded tanks and outdoor furniture made with this grade show improved surface temperature under solar irradiation compared with unpigmented LLDPE, but the part must be evaluated for weathering under ISO 4892-2:2013 if it is installed above ground. For potable-water or food-contact rotomolded parts, the final part requires certification under 21 CFR 177.1520, 21 CFR 178.3297, and any applicable NSF/ANSI 61 requirements; raw-masterbatch compliance does not automatically transfer to the molded article.

Thin-wall injection molding of LLDPE-rich compounds containing LL8350 AP requires a different thermal profile than film extrusion because the pigment particles act as nucleation sites and increase the compound's pressure-volume-temperature response during packing. Molders add 2–6 wt% masterbatch to high-flow LLDPE with a melt index of 10–20 g/10 min under ISO 1133-1:2022; the blend is processed on a 100–200 ton hydromechanical clamp machine using an 8-cavity hot-runner tool with valve gates. Barrel temperatures are set at 190–220°C, the hot runner is held 5–10°C below the nozzle, and the mold coolant is controlled at 15–30°C because white parts show dark yellow streaks when the mold surface exceeds 40°C or when the residence time in the hot runner exceeds 5 min. Injection speed is kept at 50–150 mm/s; very high injection speed creates shear heating at the gate, causing the TiO₂ particles to align and produce silver streaks at the gate blush. Shot weight is set so that the cushion remains 2–4 mm, and post-gate pressure is held at 600–900 bar for 1.5–3 s to control sink marks in the lid centre.

The main dimensional risk is anisotropic shrinkage. Under ASTM D955-23, the white compound will typically show a different in-flow and cross-flow shrinkage than the unpigmented LLDPE control; if the masterbatch letdown is allowed to drift by more than 0.5 wt%, lid curl and fitment force can fall outside specification. Molders should set gravimetric dosing alarms at ±0.3 wt% for thin-walled closures and use cavity-pressure sensors to detect lot-to-lot variation in melt viscosity. LL8350 AP is compatible with PE-rich formulations; in PP-rich closures it should not exceed 3 wt% unless weld-line strength and hinge-flex cracking are explicitly validated, because the LLDPE carrier can form a discrete polyethylene phase at the weld line and reduce notched Izod impact under ASTM D256-10. Food-contact thin-wall articles made from the PE blend are evaluated under 21 CFR 177.1520 and 21 CFR 178.3297; overall migration in the EU is tested under EN 1186-1 and specific migration is addressed under Regulation (EU) No 10/2011.

Extrusion Coating White Opacity on Brown Kraft Board

Extrusion coating lines that apply white LLDPE/LDPE layers onto brown kraft board use LL8350 AP at 8–15 wt% because the white layer must hide the brown substrate at coat weights as low as 12–25 g/m². The masterbatch is blended with a coating-grade LDPE with a melt index of 7–8 g/10 min and a density of 0.918–0.923 g/cm³; the addition of LLDPE carrier increases melt draw-down and reduces neck-in, but it also lowers thermal stability at the extreme 285–315°C melt temperatures required for adhesion to paperboard. The extruder is a 120 mm 30:1 L/D machine with a barrier screw and a screen pack of 80/100/80 mesh; melt pressure at the adapter is kept below 350 bar because higher pressure indicates a gel or pigment agglomerate buildup at the unheated feedblock. The die gap is set at 0.5–0.8 mm, the air gap at 150–250 mm, and the chill roll at 15–20°C; ozone treatment of the melt surface is applied at 0.5–1.0 L/min to raise the polar surface energy before the melt contacts the paperboard at 80–100°C.

The white layer is tested for luminous transmittance to determine hiding power; for a 15 g/m² coating, a total luminous transmittance below 25% under ASTM D1003-13 is often required to maintain a clean white cup stock appearance. If the result is below specification, increasing masterbatch above 18 wt% may create chill-roll release problems, screw slip, and smoke generation at the die; a better approach is to raise the coat weight to 18–20 g/m² or use a two-layer white-on-white structure with a thin titanium-rich tie layer. The extrusion-coated board is tested for water absorption under ISO 535 and for ply adhesion in the z-direction under TAPPI T 541. Food-service cups and frozen-food board made from this structure require compliance with 21 CFR 175.300 for resinous and polymeric coatings, 21 CFR 176.170 for components of paperboard in contact with aqueous and fatty foods, and Regulation (EU) No 10/2011 for the plastic layer.

Plate-Out on Die Lips Develops When Low-Molecular-Weight Carrier Fractions Volatilize at 215°C

White PE conduit and wire-duct processors blend LL8350 AP into natural LLDPE or LDPE at 3–6 wt% and extrude through a 68 mm 30:1 L/D single-screw line with a corrugator or vacuum sizer. The barrel profile is set at 180–210°C, and the die is held at 195–205°C; if the die is pushed to 215°C to reduce melt viscosity, low-molecular-weight fractions from the carrier and external lubricants begin to volatilize and condense on the die lips as a white or yellow deposit. This deposit re-enters the melt stream intermittently and appears as surface pits, gloss bands, or black specks on the conduit. The problem is controlled by reducing the die temperature to 200°C, increasing die-land length to 15–20 mm, and purging with a dilute white-containing compound at the end of each shift. A 120-mesh screen pack is normally sufficient; if the conduit must be weather-resistant, the final article is tested under ISO 4892-2:2013 and ASTM G155-13 for UV aging, with CIE whiteness retained above 70 after 500 h. The white conduit is designed for outdoor cable protection and ducting; electrical performance is governed by the finished conduit standard, not by the pigment dispersion alone.

WPC capstock operations processing LLDPE-based wood-plastic composites use LL8350 AP at 2–5 wt% in the outer cap layer to produce white deck boards, rail covers, and fascia profiles. The masterbatch is fed into the cap-layer extruder, typically a 55 mm 28:1 L/D conical twin-screw or parallel twin-screw machine, while the core contains wood flour, HDPE or LLDPE, coupling agent, and lubricant. The cap layer is extruded at 160–180°C melt temperature to prevent thermal degradation of the wood-filled core; at this lower temperature, the LLDPE carrier in the masterbatch melts readily and disperses the TiO₂ without requiring a high-shear mixing zone. The die temperature is held at 175–185°C; raising the die temperature above 190°C accelerates moisture release from the core layer and creates surface blisters in the capstock. Because the cap layer is only 0.5–1.5 mm thick, the masterbatch letdown must be stable within ±0.3 wt% or the deck board shows visible shade variation along the run. The TiO₂ dispersion is checked by optical microscopy on a pressed film using ISO 18553:2002; agglomerates larger than 10 µm are rejected for capstock because they appear as surface specks after co-extrusion. Weathering is tested under ASTM G155-13 cycle 1; whitening and chalking are evaluated after 1000 h, but published data for this specific grade in WPC capstock is limited. The finished profiles are tested for flexural modulus under ASTM D790-17, notched Izod impact under ASTM D256-10, and coefficient of linear thermal expansion under ISO 11359-2:1999 because the cap layer changes the thermal expansion balance and can cause end-to-end bowing if the white masterbatch is distributed unevenly.