Borealis Borstar® FB4230 is a linear low-density polyethylene film resin produced by the Borstar bimodal process. The nominal density is 0.923 g/cm³ and the nominal melt flow rate is 0.4 g/10 min at 190°C under 2.16 kg, measured to ISO 1183-2:2019 and ISO 1133-1:2022. The low melt flow rate and moderate density place FB4230 in the high-molecular-weight film-grade LLDPE envelope, which changes bubble cooling, die pressure, and seal response in downstream lines compared with conventional C4-LLDPE. The scenarios below are separated by process boundary rather than by market label, and each identifies compliance anchors, addition levels, downstream production route, and terminal product types.

Why Heavy-Duty Sack Lines Running Below a 0.4 g/10 min MFR Demand a Narrow Frost-Line Band

On three-layer and five-layer blown-film lines producing heavy-duty industrial sacks, FB4230 is run as the core layer at 75–85 wt%; LDPE with an MFR of 0.25–0.7 g/10 min is added at 10–20 wt% to stabilise the bubble, and a processing-aid masterbatch is introduced at 0.02–0.05 wt% only if die-lip build-up or melt fracture becomes visible. The downstream process is defined by grooved-feed extruders with screw diameters of 75–120 mm and L/D ratios of 30:1–33:1, annular die diameters of 350–600 mm, die gaps of 2.0–2.5 mm, blow-up ratios of 2.2:1–2.8:1, and melt temperatures of 195–220°C. Internal bubble cooling is used on lines where total film gauge must remain within ±5%; without IBC, long-term gauge variation in 100 µm sacks increases because the low-MFR resin retains heat longer in the frost-line zone. Output on such lines ranges from 450 kg/h to 650 kg/h, but the rates are constrained by cooling capacity rather than torque. The critical processing boundary is frost-line height: with an MFR of 0.4 g/10 min, the frost line is normally held at 6–8 die diameters; a movement of even ±10 mm can alter MD/TD tear balance and reduce dart impact on high-output lines. Screen-changer pressure behaviour is another operational indicator. On a 90 mm grooved-feed extruder with an 800 µm filter pack, pressure at the breaker plate is typically maintained below 400 bar; when regrind content exceeds 15 wt%, pressure drift of 20–40 bar over a 7-day run has been observed, and the resulting shear-heating shift changes melt temperature at the die lip. Mechanical acceptance is anchored to ISO 527-3:2018 for tensile yield and elongation, ASTM D1709-16a Method A for dart impact, and ASTM D1922-15a for Elmendorf tear; the packaging itself must satisfy heavy-metal and source-reduction provisions under EU Packaging and Packaging Waste Directive 94/62/EC and REACH 1907/2006. Terminal products include 80–200 µm heavy-duty sacks for polymer resin, carbon black masterbatch, precipitated silica, construction chemicals, salts, and mineral fillers, as well as heavy-duty drum liners and inner liners for flexible intermediate bulk containers.

In coextruded temporary protective films for anodised aluminium profiles, polycarbonate glazing, stainless steel sheet, and PVC window sections, FB4230 is placed in the structural core at 60–85 wt%; the skins are compounded from EVA or POE resins at 15–40 wt% to lower heat-seal initiation and supply controlled cling without a pressure-sensitive adhesive masterbatch. The production route is three-layer cast-film extrusion with slot die widths of 450–1,200 mm, die gap 0.8–1.5 mm, chill-roll temperature 18–28°C, and line speed 80–180 m/min; cooling capacity generally limits throughput because the skin layers are more sensitive to chill-roll contact and sticky web transfer. Peel force is determined by the converter against the specific substrate and surface roughness rather than by a single universal standard; where an internal specification is required, the film is evaluated by tensile properties under ISO 527-3:2018 and puncture resistance under ASTM D5748-23 or equivalent substrate-specific methods. Compliance for electrical and electronic applications is driven by RoHS Directive 2011/65/EU Annex II, while REACH 1907/2006 screening applies to all commercial shipments; phthalate and organotin restrictions are mandatory when the film is used on coated metal or glass in enclosed spaces. Terminal products include 30–80 µm protective masking for aluminium composite panel cladding, temporary surface-protection film for stainless steel sheet during bending and laser cutting, and protective film for rigid glazing sheets during transport and installation.

For three-layer agricultural film programmes that alternate silage-wrap and greenhouse-cover formats on the same die, FB4230 is used as the structural core at 40–60 wt% of total formulation; the outer skins carry the functional additive load. UV-stabiliser masterbatch is added at 4–8 wt% to the exposed layer, anti-fog masterbatch at 1–3 wt% where condensation control is specified, and titanium-dioxide white masterbatch at 6–12 wt% for high-reflectance greenhouse cover and basal silage wrap. The manufacturing route is blown-film coextrusion with die gap 1.8–2.4 mm, blow-up ratio 2.2:1–2.6:1, melt temperature 210–230°C, and total thickness 20–30 µm for silage bale wrap or 150–200 µm for multi-season greenhouse cover. A process boundary emerges on lines that switch from silage to greenhouse formats without purging the white masterbatch thoroughly; carrier-polymer residues from the white masterbatch can produce gel-like specks in transparent greenhouse film, and the die gap should be opened by 0.2–0.4 mm during the transition to compensate for the higher additive loading. Compliance is anchored to EN 13206:2017 for agricultural covering films and EN 14932:2006 for stretch silage wrapping film; where the film contacts animal feed, the converter applies Regulation (EC) No 183/2005 feed-hygiene controls and REACH 1907/2006. Published data for FB4230 as a high-tack silage cling layer is limited; the cling skin is normally a different LLDPE or EVA grade. Terminal products include silage bale wrap, high-reflectance greenhouse cover, low-tunnel film, and side-film for multi-bay polytunnels.

When FB4230 Is Substituted into Form-Fill-Seal Laminate Webs Without Re-Setting the Seal Jaw Pressure

Cast-film laminating lines producing sealant webs for BOPP/BOPET/aluminium foil laminates use FB4230 at 55–80 wt% in the sealant layer, diluted with EVA containing 12–18% vinyl acetate or with LDPE at 20–45 wt% to reduce seal initiation and improve hot tack. The cast line is specified with a slot die 1,200–1,600 mm wide, sealant-layer thickness 20–60 µm, matte chill-roll finish Ra 0.4–1.0 µm, and line speed 100–200 m/min. Hot-tack and seal-strength behaviour is measured under ASTM F1921-12 and ASTM F88/F88M-21; seal initiation for a 40 µm cast sealant web is typically checked from 100–115°C, but the actual heat-seal window shifts when the web is laminated to aluminium foil or BOPET because the upper laminate layer changes heat transfer into the sealant. The critical processing risk is substitution without re-setting the sealing equipment. If a converter changes from a conventional C4-LLDPE sealant grade to FB4230 at 50–80 wt% and leaves seal jaw pressure, dwell, and anvil temperature unchanged, leaker formation in vertical form-fill-seal packaging shifts to corner gussets and is detected only by water-bath or dye-penetration testing because the higher-molecular-weight resin requires longer chain diffusion at the seal interface. Barrier and migration compliance for food use is anchored to FDA 21 CFR 177.1520(c), EU Regulation No 10/2011 Annex I with an overall migration limit of 10 mg/dm², and EC Regulation 2023/2006 for good manufacturing practice; where the laminate is used for dairy or high-fat products, the converter must perform migration testing with the final printed and adhesively laminated structure, not the cast web alone. Terminal products include dry-mix pouches, frozen-food flow-wrap, coffee valve bags, lidding films for dairy multipacks, and pillow pouches for powdered drinks.

In five-layer stretch-hood lines, FB4230 is assigned to the central load-bearing layer at 50–70 wt% of total gauge; the two skin layers carry cling and slip packages while the tie layers are selected for layer-to-layer adhesion. Total film thickness is 80–120 µm, and the film is produced by blown-film or cast-film coextrusion followed by transverse welding into hoods. On a 1,200 mm annular die, throughput is usually limited by blocking and creasing on the collapsing frame rather than by extruder torque. Compliance is driven by REACH 1907/2006 and destination-market packaging regulations; there is no single EU harmonised product standard for stretch-hood film, so load retention is generally verified by instrumented stretch-hood testing on the purchaser’s pallet geometry. Published data for FB4230 in sustained high-stretch hood applications is limited; the selected gauge should be validated on the target hooder under 10–15% nominal pre-stretch rather than extrapolated from pallet-wrap film. Terminal products include stretch hoods for palletised household appliances, bagged cement, insulation rolls, and building panels.