Why Does Pre-Stretch Performance Depend on Secondary Comonomer Ratio More Than Resin Density?

Braskem LF320 is a butene-based linear low density polyethylene with a nominal melt flow rate of 2.7 g/10 min at 190 °C under 2.16 kg load according to ISO 1133-1 and nominal density 0.918 g/cm³ under ASTM D792. In machine-grade pallet stretch film, the resin is compounded at 65–80 wt% as the primary structural component, while a C6 or C8 metallocene LLDPE at 10–25 wt% shifts the short-chain branch distribution toward higher elongation at break under ASTM D882. The pre-stretch threshold is governed more by the secondary comonomer ratio than by base resin density because the butene branch of LF320 alone does not provide sufficient transverse-direction tear resistance at pre-stretch levels above 200% on gusseted loads with sharp corner radii. Without the secondary comonomer, machine-direction strain hardening concentrates stress at the pallet corner and propagates tears that cannot be corrected by increasing film gauge alone.

Typical converting configuration uses a single-screw extruder with 30:1 to 36:1 L/D and a barrier screw equipped with a Maddock mixing section. Barrel temperatures are ramped from 180 °C at the feed throat to 250 °C at the adapter, with the die maintained at 250–260 °C to depress melt fracture at line speeds of 300–600 m/min. The die gap is set at 0.5–0.7 mm; a vacuum box or air knife with differential pressure in the 0.5–1.5 kPa range forces the web onto a chill roll held at 18–24 °C. Within this window, crystalline orientation remains low enough to permit secondary stretch without rapid stress whitening. If the chill roll temperature is raised above 28 °C, the film blocks at wind-up because low-temperature crystallization is incomplete; if the air gap exceeds 80 mm, neck-in and edge thickness variation exceed automatic profile control limits.

Formulation additions comprise polyisobutylene tackifier at 1.0–2.5 wt%, erucamide at 300–800 ppm, and silica antiblock at 1,000–2,500 ppm. Cling force is measured under ASTM D5458 at 23 °C, with a production target range of 50–150 g for film thickness of 20–23 µm; unwind noise is controlled by differential cling between the two faces of the cast film rather than by raising tackifier level. Higher tackifier loadings may restore cling after roll storage but degrade load retention force under ASTM D4649 because the film layers slide relative to each other instead of distributing load into the gusset. Published data for this specific LF320 configuration is limited; converter trials are required to correlate formulated cling force with wrapper pre-stretch gearing and load cell tension.

Operational boundary: LF320-based core layers are not recommended for single-layer hand stretch film with continuous hand-applied tension above 70% elongation because the butene backbone exhibits lower puncture propagation resistance than C8-based films under ASTM D5748. The resin is compatible with post-consumer recycle streams, but inconsistent recycle melt indices in the 1–3 g/10 min range can introduce gel particles that appear as specks in the film; a downstream melt filter with 100 mesh or finer is therefore maintained during compounding.

Retail cling film compounding with LF320 requires a separate additive package because the neat resin carries no antifog surface treatment and no organoleptic neutral flavour barrier. A common formulation band uses 75–88 wt% LF320, 5–12 wt% ethylene-vinyl acetate copolymer with vinyl acetate content of 12–18%, 1–3 wt% polyisobutylene tackifier, 500–1,200 ppm erucamide, and 1,000–3,000 ppm synthetic silica antiblock. The EVA component reduces crystallite melting point and raises finger tack; the silica prevents roll blocking after the film ages in warehouse conditions above 30 °C.

The film is cast at 9–12 µm thickness, with extruder melt temperature held at 230–250 °C and chill roll at 20–24 °C to maintain haze below 2.0% under ASTM D1003. Gloss at 60° is maintained above 80 GU under ASTM D2457 by using a mirror-finish chill roll and controlling die lip deposits. Coefficient of friction is adjusted to 0.20–0.40 under ASTM D1894 so that the web can be cut and wound on a high-speed household roll winder without telescoping. The low crystallinity of LF320 at 0.918 g/cm³ controls softness and drape more directly than tackifier content alone.

Food contact status is governed by FDA 21 CFR 177.1520(c) for olefin polymers and by EU 10/2011 for plastic materials intended to come into contact with food; the overall migration limit is 10 mg/dm² for simulant D2 under the specified time and temperature. Dual-use additives such as erucamide are subject to positive-list restrictions and specific migration limits; converters must obtain lot-specific statements from the additive supplier. The film is not suitable for oven bags or hot-fill above 80 °C because dimensional change accelerates under film stress. Sharp edges on foam trays puncture the unsupported web below 9 µm; therefore retail tunnel-pack applications generally require coextruded skins with at least 15 µm total film.

UV Stabilizer Loading and Film Embrittlement in Silage Storage

Silage bale wrap produced on cast film lines uses LF320 let down at 80–90 wt% with an ultraviolet-stabilised C6 or C8 LLDPE at 5–10 wt% and a masterbatch comprising hindered amine light stabilizers at 0.3–0.8 wt% active concentration. Black grades add carbon black at 2.0–3.0 wt%; white grades incorporate rutile TiO₂ at 4.0–7.0 wt% and require a dispersion quality check because pigment agglomerates larger than 20 µm create pinholes under bale tension. The film is exposed to UV, silage fermentation acids, and constant contact pressure; the failure mechanism is not tensile yield but embrittlement along machine-direction orientation. Standard agricultural film specifications such as EN 13207 require minimum dart impact and tear retention after weathering, so a single-step cast film without sufficient HALS will crack during the storage season.

Target film thickness is 25 µm, with tolerance band ±2 µm across the web because bale wrap coverage claims are based on weight per bale. The cast line uses a die gap of 0.6–0.8 mm, chill roll at 22–28 °C, and line speed up to 300 m/min. The film is corona treated in-line to 38–42 mN/m on one surface for ink and label adhesion but not on both faces; double-sided corona raises tack and blocks during roll storage. Roll length is slit to widths of 250 mm, 500 mm, or 750 mm depending on bale wrapper geometry and pre-stretch ratio of 55–70%. Unlike industrial pallet film, agricultural bale wrap is applied in multiple continuous rotations; it must retain cling after stretching without developing unweldable wrinkles at the bale shoulder.

Fermentation acids under the film surface alter the surface coefficient of friction; therefore cling retention is measured after a 7-day acid exposure simulation at 30 °C using dilute acetic acid. The film must retain at least 70% of initial cling force under ASTM D5458 after acid contact, according to typical agricultural film technical agreements. The processing boundary for LF320 in this application is the carbon black dispersion window: raising carbon black above 3.0 wt% reduces elongation at break below accepted silage film minimums, while lowering it below 2.0 wt% reduces UV opacity to a level that cannot guarantee 12-month outdoor storage. Converters should verify weathering performance according to ISO 4892-2 with a radiant exposure agreed with the bale wrap brand owner.

In coextruded surface protection film for stainless steel sheet and polycarbonate panels, LF320 serves as the backing layer at 65–80 wt% of the film structure. The adhesive skin is compounded separately from maleic anhydride-modified polyolefins or acrylic copolymers and is not based on LF320. The backing layer requires a melt flow of 2.5–2.8 g/10 min to match skin layer viscosity in the feedblock; viscosity mismatch above 10% at melt temperature causes layer distortion and wavy edges. The film is cast at 30–50 µm total thickness, with the skin layer occupying 10–15% of the total. A vacuum box pins the web to a matte chill roll at 18–22 °C, producing sufficient contact clarity to inspect substrate colour without changing the surface gloss of the protected part.

Peel force is the critical terminal property. For temporary protection of polished aluminium, the film is formulated to peel at 0.05–0.15 N/25 mm, while for deep-drawing stainless steel protection, peel force is increased to 0.20–0.45 N/25 mm using tackifier in the skin layer. Measurement is made at 180° peel angle under ASTM D3330/D3330M after 24 h dwell at 23 °C and 50% relative humidity. The backing must not leave adhesive residue; LF320 contributes low surface migration of low-molecular-weight species because it is a copolymer with narrower molecular weight distribution than high-autoclave LDPE. Amine-containing cleaning agents can soften the backing layer surface and should be excluded from converter wash tanks.

Regulatory constraints for surface protection films are driven by end-use metal fabrication rather than food contact. REACH Annex XVII restrictions on polycyclic aromatic hydrocarbons and heavy metals apply to pigment masterbatches; RoHS 2011/65/EU applies when protective film is used on electronic display parts. The film should not be exposed to outdoor UV for more than 6 weeks because adhesion build in the skin layer can exceed removal thresholds and leave ghosting on the substrate. Converters report that storage above 40 °C accelerates migration of tackifier to the backing interface and produces blocking; pallets should therefore be kept in shaded warehouses.

When Reverse-Printed Laminates Require a Low-Gel Cast Skin

Reverse-printed structures for confectionery and dry-food laminates place LF320 in the sealant ply, typically at 60–75 wt% of the sealant layer with LDPE at 15–25 wt% and an antiblock/slip package. The sealant layer is cast onto the inner face of a printed polyester or biaxially oriented polypropylene web; the function is not structural strength but low seal initiation temperature and isolated seal through hot-tack. The butene branch of LF320 lowers the differential scanning calorimetry melting peak to 122–124 °C, while the LDPE component maintains seal bar release above 90 °C. The film is processed at 20–35 µm, with melt temperature limited to 240–250 °C to avoid gel formation. Gels larger than 100 µm create optical defects in the print area and are unacceptable for registered packaging.

Seal strength is recorded under ASTM F88/F88M at 150 °C seal bar temperature, 0.35 MPa seal pressure, and 0.1 s dwell. Seal initiation temperature is the lowest bar temperature producing 2.0 N/15 mm seal strength; LF320-rich formulations generally sit in the 95–105 °C range, but published data for this exact cast film configuration is limited. Hot tack is the critical process parameter for vertical form-fill-seal operations, where the still-warm seal must hold 200–500 g of product without creep. The butene-based LF320 sealant exhibits lower hot tack than C6 or C8 metallocene grades, so form-fill-seal lines with fill weights above 500 g require seal layer thickness above 35 µm or a coextruded C6-rich seal layer.

Substrate lamination uses a solvent-free polyurethane adhesive between the printed web and the LF320 cast film. If the cast film is not corona treated above 38 mN/m, adhesive wet-out becomes uneven and bond strength fails under ASTM F904. The film must be wound with low tension, below 0.5 N/cm, to prevent inner-slip migration of erucamide onto the seal surface over roll storage; erucamide bloom is desirable on the outer surface but not on the reverse surface because it can reduce seal bond consistency. The application is not suitable for retort or hot-fill environments above 90 °C because seal creep approaches the Vicat softening point of the LF320 fraction and may open under product head pressure.

High-opacity cast film for hygiene backsheet lamination uses LF320 at 12–18 g/m², with rutile TiO₂ at 5–8 wt%, blue and violet colour masterbatch at 0.5–1.5 wt%, and a processing aid to reduce melt fracture. The film is cast against a matte chill roll and subsequently bonded to a polypropylene spunbond nonwoven using hot-melt adhesive. The backing film must resist lint penetration and provide a moisture barrier; the crystalline polyolefin structure at 0.918 g/cm³ yields water vapour transmission values typical of LLDPE cast film but not breathable-film levels. This distinguishes the product from breathable hygiene films, which require filled microporous layers with calcium carbonate loadings above 40 wt%.

Tensile strength is measured under ASTM D882 at 300 mm/min jaw separation; the film should retain at least 15 N/cm in the machine direction after corona treatment. Corona discharge is maintained at 38–40 mN/m on the adhesive side, but the rest of the film surface must not exceed 42 mN/m to avoid inconsistent adhesive penetration. Process control in the cast line focuses on die lip build-up from TiO₂ agglomerates; a screen pack of 100/150/200 mesh and a static mixer are installed upstream of the die. Failure to control agglomerates above 25 µm creates visible pinholing in a 12 g/m² film.

Compliance for hygiene applications follows ISO 10993-5 for cytotoxicity when the film is sold to medical hygiene converters; general market films conform to REACH and European Union consumer product safety regulations. The film is not intended for sterilisation by gamma irradiation above 25 kGy because the butene copolymer can lose elongation and develop odour from oxidative species; ethylene oxide or electron beam processes are preferred if sterilisation is required. Cast film rolls are stored at 10–30 °C and below 60% relative humidity to maintain consistent unwind tension for the adhesive lamination line.