Why Cast Film Lines Exhibit Melt Pressure Drift with VLDPE-Rich Formulations

On five-layer cast stretch-film lines running payloads of 12–23 µm machine wrap, Sinopec Tianjin VLDPE TJVL-084Z is introduced in the outer cling layer at 60–85 wt% and in the longitudinal core layers at 20–40 wt%, with the balance of each layer typically carried by a C6/C8 linear low-density polyethylene and, in the cling layer only, 1.5–3.0 wt% polyisobutylene tackifier. Melt temperature at the die is maintained between 240°C and 270°C, the slot die lip gap is held at 0.4–0.8 mm, and the air gap from die exit to chill roll is set between 120 mm and 200 mm to suppress draw resonance while still allowing cling development through surface roughness transfer. The chill roll temperature is normally logged at 15–25°C; lower roll temperatures increase surface gloss but also increase condensation risk and subsequent tackifier migration onto the roll surface after multi-hour runs. Compliance for the finished industrial stretch wrap is judged against ASTM D5458-95 for peel cling, ASTM D882-18 for tensile modulus and elongation at break in both machine and transverse directions, and ASTM D5748-95 for protrusion puncture resistance. The weight percentage split between TJVL-084Z and the LLDPE carrier is adjusted lot-to-lot only after melt pressure and chill roll amperage have been trended for at least three consecutive shifts, because the 2.16 kg melt mass-flow rate value alone does not capture shear-thinning behaviour relevant to cast-film lip exit stability.

Production-scale failure data from this film class indicate that raising TJVL-084Z in the core layers above 40 wt% produces a characteristic edge-gauge oscillation of ±2–4% when the die temperature differential between the centre and edge zones remains fixed, because the melt temperature sensitivity of VLDPE relative to the C8 LLDPE alters transverse die-swell distribution. The corrective action is not uniformly to increase die edge-zone setpoints, but to reduce the core-layer VLDPE fraction to 35 wt% or to lower the overall line speed by 5–10% until the chill roll torque signal stabilises. Terminal product types from this segment include 5–8 µm hand wrap, 12–23 µm machine wrap, 12–18 µm pre-stretched wrap with 2.0:1–2.5:1 prestretch ratio, and anti-static or coloured containment wrap for racking and transit loads. For the European Economic Area, the final article falls under REACH Regulation (EC) No 1907/2006 and should be verified against any candidate list SVHC content above 0.1% w/w.

Direct food-contact sealant layers for fresh-cut produce, frozen vegetables, and bakery flow-wrap are formulated with TJVL-084Z where a low seal-initiation temperature and a broad hot-tack window must be combined with high interlayer adhesion to EVOH or polyamide barrier layers. In a three-layer blown-film structure, the sealant layer is typically loaded at 60–90 wt% TJVL-084Z, with 10–25 wt% C4 or C6 LLDPE and 5–15 wt% of a silica or zeolite anti-block masterbatch; the sealant layer itself represents 15–25% of total film thickness, while the barrier and structural layers account for the remainder. Processing is carried out on three-layer blown-film dies with a die gap from 1.0 mm to 2.5 mm, a blow-up ratio of 2.2:1–2.8:1, and a frost-line height controlled between 4 and 7 die diameters; the extruder barrel temperatures are profiled from 165°C in the feed section to 225°C at the die, and the film gauge is controlled to ±5% or better using either segmented die lips or automated internal bubble cooling. For European food-contact compliance, the finished film is assessed under Commission Regulation (EU) No 10/2011 with the overall migration limit of 10 mg/dm² for the intended food simulant, and in the United States under 21 CFR 177.1520(c) for olefin polymers used in contact with food, subject to the end-use temperature and fatty-food service conditions specified in each regulation. Terminal film types include modified-atmosphere pillow packs, lidding film for polypropylene trays, frozen vegetable pouches, and fresh-cut salad bags. Because TJVL-084Z alone does not provide oxygen barrier, the formulation requires a separate EVOH or polyamide layer for high-barrier applications, and the absence of an internal adhesive layer will produce delamination at the barrier interface during heat sealing.

Heavy-Duty Sack Film: Dart Drop, Seal Integrity, and Bulk Chemical Fill Rigors

For polymer pellet sacks, flexible intermediate bulk container liners, and bulk fertiliser sacks that must resist dart drop failures at low temperatures, TJVL-084Z is compounded at 10–40 wt% of the film blend, generally in the middle and sealant layers of a three-layer coextrusion rather than in a monolayer, to raise low-temperature dart impact without sacrificing bubble stability on high-output blown-film towers. The film is produced on high-output blown-film lines equipped with internal bubble cooling and a die gap from 1.8 mm to 2.5 mm, a blow-up ratio between 2.0:1 and 2.8:1, and a total thickness from 80 µm to 150 µm; final heat sealing at the sack bottom and top is typically performed at 130–160°C with dwell times of 0.5–1.5 s and pressure of 0.2–0.6 MPa. The finished film is evaluated using ASTM D1709-20 Method A or B for dart drop impact, ASTM F88/F88M-21 for seal strength, ISO 527-3:2018 for tensile properties, and ISO 1183-1:2019 for density verification. Terminal products include liners for FIBCs carrying polymer granules, valve sacks for powdery construction chemicals, and heavy-duty sacks for salt or fertiliser; compliance with dangerous goods packaging requirements normally attaches to the outer bag or liner combination under the applicable UN performance packaging test methods, while the VLDPE liner itself is specified by its mechanical test values and the supplier's food-contact or chemical-resistance declarations where relevant. A recurring production limit on this application is that TJVL-084Z additions above 35 wt% in the middle layer reduce bubble stability under fluctuating ambient humidity, producing creases and gauge bands that lower the stated dart drop by more than the resin alone would predict; pre-drying of the polymer and the anti-block masterbatch at 60–70°C for 2–4 h is recommended when relative humidity exceeds 60%.

Agricultural silage wrap and multi-season greenhouse covers use VLDPE-containing interlayers to retain puncture resistance when field temperatures fall below −20°C; in this segment, TJVL-084Z is incorporated at 15–35 wt% into the middle layer of a three-layer blown-film structure, with the balance split between EVA and a C8 LLDPE. For bale wrap the total film thickness is commonly 18–45 µm, while greenhouse cover and low-tunnel film runs from 80 µm to 200 µm. The extrusion process uses a three-layer blown-film line with a blow-up ratio of 2.0:1–2.6:1, and the UV stabiliser package is pre-compounded in an LLDPE carrier at 1.5–3.0 wt% masterbatch addition rather than direct powder dosing, because direct addition of hindered amine light stabilizers produces gel specks and lowers dart impact. Product standards applicable to the agricultural film include EN 13206:2017 for thermoplastic films used in agriculture and horticulture, supplemented by ASTM D882-18 or ISO 527-3:2018 for tensile properties and ASTM D1709-20 for dart drop; for silage film, the customer specification typically requires puncture resistance, elongation at break, and UV-weathering retention after artificial weathering according to the relevant annex of the standard. Terminal product types include round-bale silage wrap, square-bale silage film, greenhouse cover film, and low-tunnel film. The service limitation is that TJVL-084Z is not a UV-stabilised grade, so all outdoor film must contain a separately supplied stabiliser masterbatch, and the absence of such a package will result in rapid embrittlement under solar load.

When VLDPE Replaces LLDPE in Geomembrane Flexibility Layers

In multi-layer geomembranes for landfill capping, pond liners, and floating covers, TJVL-084Z is compounded into the flexibility layer at 20–50 wt% with the balance a LLDPE or HDPE carrier, depending on whether the coextruded sheet must retain HDPE-like stiffness or exhibit greater elongation before yield. The sheet is produced by flat-die coextrusion at die temperatures from 180°C to 220°C and total thickness from 0.75 mm to 1.5 mm; polished or textured rolls control surface morphology, and the line is typically fitted with thickness scanners operating at ±5% or better. For tensile evaluation, ASTM D6693-20 is applied to the geomembrane; tear resistance is commonly measured using ASTM D1004-13, and density may be verified by ISO 1183-1:2019. Project specifications often invoke GRI-GM13 for HDPE geomembrane properties; however, a VLDPE-containing flexibility layer is not automatically equivalent to an HDPE geomembrane under GRI-GM13, and the layer-specific tensile and ductility values must be assessed separately for the liner as-built. Terminal products include landfill cap flexibility layers, floating covers for reservoirs, aquaculture pond liners, and secondary containment liners for mildly aggressive aqueous solutions. The chemical compatibility boundary is significant: VLDPE-based layers are less resistant than HDPE to aromatic hydrocarbons, chlorinated solvents, and strongly oxidising fluids; for such service, the liner design should be qualified with immersion testing under the specific site chemistry, and published data for long-term oxidative induction time of this exact grade in buried geomembrane service remain limited.

Where post-industrial PE film recyclate exhibits reduced tear-propagation resistance after repeated pelletising, TJVL-084Z is metered into the reclaim stream at 5–15 wt% during twin-screw compounding to restore inter-lamellar energy absorption without reducing the recycled blend’s melt strength below the level required for subsequent blown-film extrusion. Compounding is carried out on a co-rotating twin-screw extruder with screw diameter from 32 mm to 56 mm and an L/D ratio from 32:1 to 40:1; the melt temperature is controlled at 210–230°C, and the melt is passed through screen packs from 100 µm to 250 µm before underwater pelletising. The modified recyclate is then tested under ASTM D638-14 for tensile modulus and elongation at break, ISO 178:2019 for flexural modulus where stiffness is critical, and ISO 1133-1:2022 for melt mass-flow rate. Compliance for the final article is evaluated under REACH Regulation (EC) No 1907/2006; polymer substances are not subject to registration under Article 2(9), but downstream users remain responsible for verifying any intentionally added substances and any SVHC content above 0.1% w/w. Terminal products from this compounding route include refuse bags, garden pots, construction film, and impact-modified crates or storage bins where a marginal loss of stiffness is acceptable. Above 15 wt% TJVL-084Z addition, tensile modulus of the recyclate blend typically declines to a level that may fall below the customer’s specified ASTM D638 secant modulus value; this boundary should be established on the intended production twin-screw line rather than extrapolated from laboratory batch mixers.