Direct substitution of PBAT for LDPE on a high-output T-shirt carrier bag line is not a resin-grade exchange, because the two polymers differ in melt rheology, density, thermal degradation mechanism, and heat-seal response. A typical carrier bag LDPE resin has a melt mass-flow rate of 0.2–0.8 g/10 min at 190 °C under a 2.16 kg load when tested to ISO 1133-1:2022, and a density of 0.918–0.924 g/cm³ when tested to ISO 1183-1:2019. Commercial PBAT film grades more commonly show melt flow rates of 2.5–5.0 g/10 min under the same conditions and densities of 1.25–1.27 g/cm³. The density difference alone increases the basis weight of a 20 µm film from approximately 18.4 g/m² for LDPE to approximately 25.2 g/m² for PBAT, a mass penalty of about 36–37% at constant thickness. This imposes immediate limitations on high-cycle bag weight specifications and cost structures. The melting peak of PBAT, measured by differential scanning calorimetry under ISO 11357-3:2018, typically lies between 110 °C and 125 °C, overlapping with LDPE, but the polyester backbone introduces ester linkages that undergo hydrolysis if moisture is not controlled; LDPE is not subject to this degradation route. The practical consequence is that PBAT requires predrying, barrier screw designs with lower compression ratios, and bubble-stability settings that differ from LDPE optimisation routines. Processors must therefore evaluate PBAT as a separate blown film material rather than a drop-in additive package change.

What Restricts Bubble Stability in PBAT High-Cycle Film Lines?

On a high-cycle blown film line producing T-shirt bags at take-off speeds between 80 m/min and 150 m/min, the bubble is continuously tensioned by the nip and collapsing frame. LDPE has high melt strength due to long-chain branching, allowing stable deformation at blow-up ratios of 3.0:1 to 4.0:1 and high-stalk bubble geometries. PBAT, however, has limited strain hardening and lower elongational viscosity. Single-screw extrusion trials on a 45 mm grooved-feed extruder with 30:1 L/D and a 150 mm spiral mandrel die indicate that stable PBAT bubbles are generally maintained only within a blow-up ratio of 2.0:1 to 3.0:1; above 3.0:1, lateral oscillation and periodic draw resonance become observable in the frost-line region. Frost-line height must be reduced from 5–8 die diameters, typical for LDPE, to 3–6 die diameters for PBAT, and the chilled air ring output must be increased by 10–20% to rapidly set the polyester melt. A low-stalk bubble configuration with a short neck and an internal bubble cooling system is recommended by blown film equipment suppliers for PBAT; however, published production data for PBAT at speeds above 120 m/min remain limited. The lower melt strength also means that melt temperature must be kept in the 185–205 °C range to avoid both bubble sag and thermal scission. Increasing melt temperature above 210 °C does not produce enough melt strength improvement and instead accelerates molecular weight loss. Bubble cages with ultrasonic width sensors reduce lateral movement because PBAT bubble walls are more sensitive to external air currents than LDPE, but this does not fully compensate for the loss of strain hardening.

Before the resin reaches the die, moisture and screw geometry control the available molecular weight. PBAT pellets must be dried to a residual moisture content below 250 ppm, and preferably below 150 ppm, using a desiccant dryer with a dew point no higher than -40 °C and a residence time of 4–6 h at 70–80 °C. Failure to dry to this level causes hydrolysis in the melt, measurable as a loss in melt viscosity and an increase in melt flow rate of more than 0.5 g/10 min over a single production shift. The screw configuration should be changed from a high-shear LDPE barrier screw with compression ratio 3.0–4.0 to a lower-shear design with compression ratio 2.0–2.5; a Maddock mixer or intensive dispersive mixing section should be removed or replaced with a distributive cavity-transfer mixer, because localised shear heating above 230 °C initiates chain scission. Barrel temperatures should be set to a flat-to-reverse profile, for example 140–150 °C in the feed zone, 150–165 °C in the compression zone, and 165–180 °C in the metering zone, while the adapter and die are maintained at 175–190 °C. These zones are lower than the typical LDPE profile of 160–200 °C and are intended to hold melt temperature below 205 °C. The melt pressure at the die entry for PBAT is commonly observed in the range 15–25 MPa, which is lower than the 20–35 MPa typical of LDPE due to higher melt flow. The screen pack should use 100–250 µm screens with a breaker plate that minimises dead zones; stagnant melt in the adapter or die lip can produce yellow specks and gel-like defects within 30–60 min if any unheated or low-flow region exists.

Degradation Is Initiated by Heat and Moisture, Not by Chain Branching

Thermal degradation of PBAT under extrusion conditions proceeds primarily through ester pyrolysis and hydrolysis, not through the radical chain branching that dominates LDPE degradation. The onset of measurable molecular weight loss for PBAT occurs near 230 °C in the absence of moisture, but shifts lower as moisture content increases; at 250 ppm moisture, hydrolysis can reduce number-average molecular weight by 20–40% within the residence time distribution of a 30:1 L/D extruder. Therefore the maximum melt temperature must be controlled at 200–210 °C, and the maximum barrel residence time should not exceed 3–5 min. Typical high-cycle LDPE operations tolerate melt temperatures of 210–230 °C and residence times up to 6 min; PBAT does not retain melt viscosity under those conditions. Processors should monitor melt pressure and motor load instead of barrel temperature alone, because pressure fluctuations in the die entry are an early indicator of viscosity loss. Thermal gravimetry under ISO 11358-1:2022 shows that PBAT has a primary decomposition onset around 350–380 °C, but processing stability is lost well below that onset due to ester exchange and water generation; the activation energy for thermal degradation in PBAT is reported in the range 150–200 kJ/mol depending on molecular weight and catalyst residues. No stabiliser package used in LDPE extrusion, such as phenolic antioxidant plus phosphite, is directly transferable to PBAT without verifying compatibility; some phosphate-based stabilisers may accelerate hydrolysis, and amine-based additives must be avoided because they promote transesterification and colour formation. Published data for stabilised PBAT extrusion at high throughputs above 150 kg/h is limited; processors should qualify any masterbatch through rheological and gel-count trials before line-scale adoption.

Die design and bubble geometry must be altered rather than inherited from LDPE operations. A spiral mandrel die with a die gap of 0.8–1.2 mm used for LDPE carrier films may generate excessive shear and surface roughness when processing PBAT; widening the die gap to 1.2–1.8 mm reduces shear rate and stabilises the melt surface, but also increases gauge variation at high take-off speeds. Die land length ratios of 10:1 to 15:1 are appropriate for PBAT to balance pressure generation and shear heating. The bubble is typically run with a blow-up ratio of 2.0:1 to 2.8:1, and the layflat width must be recalculated because PBAT density changes the basis weight but not the layflat calculation. Internal bubble cooling is recommended in high-output lines because the lower melt strength can be compensated by a shorter frost line and faster cooling; dual-lip air rings with chilled air at 10–20 °C and high lip angle improve bubble stability. Electromechanical bubble cages with ultrasonic sensors reduce lateral movement because PBAT bubble walls are more sensitive to external air currents than LDPE. For a 150 mm die producing 18–25 µm carrier film, the line speed is frequently limited to 50–100 m/min for PBAT, whereas the same line can sustain 100–150 m/min with LDPE. Above 100 m/min, melt fracture, gauge bands, and bubble pulsing become frequent; using a processing aid masterbatch at 0.5–1.0 wt% has been reported in some pilot trials to extend the stable window, but published data for high-cycle T-shirt bag lines is limited.

When PBAT Is Dry-Blended with Calcium Carbonate or LDPE: Phase Morphology and Processing Limits

Dry blending of PBAT with 10–30 wt% calcium carbonate or with 20–50 wt% LDPE is often attempted to recover density and cost or to improve melt strength, but the resulting phase morphology creates new processing restrictions. PBAT and LDPE are thermodynamically incompatible in the melt; without a compatibiliser, scanning electron microscopy of cryofractured films typically reveals discrete LDPE domains of 1–10 µm within the PBAT matrix at 20–30 wt% LDPE. This morphology may improve bubble stability slightly by increasing zero-shear viscosity, but it also causes gauge bands and reduced dart impact when the LDPE domain size exceeds 10 µm. Calcium carbonate, often added at 10–30 wt% to reduce cost and improve stiffness, has a density of 2.70 g/cm³ and raises the compound density even further; the elongation at break measured under ISO 527-3:2018 falls from 500–900% for unfilled PBAT film to 200–400% at 20 wt% calcium carbonate. The filler must be surface-treated with stearic acid or a suitable coupling agent; untreated calcium carbonate increases melt pressure and accelerates haze. On a high-cycle line, filler agglomerates can block the screen pack and create die-lip build-up within 4–8 h of continuous operation. Dry blending pure PBAT with LDPE is not recommended as a route to full compatibility unless melt strength and cost are prioritised over biodegradability; in that case a coextruded structure with a thin PBAT skin is more processable. Published data for ternary blends of PBAT, LDPE, and calcium carbonate on high-speed carrier bag lines remains limited.

Mechanical property translation from LDPE to PBAT must be evaluated at the same gauge, and preferably at equal basis weight, because the density penalty changes mass per bag. In tensile testing under ISO 527-3:2018 or ASTM D882-18, PBAT carrier film typically exhibits tensile modulus values of 50–120 MPa in both machine and transverse directions, compared with 150–300 MPa for LDPE carrier film; this lower modulus reduces bag stiffness and may cause unstable stacking on automatic wicketting lines. Tensile strength at break is usually 20–35 MPa for PBAT and 20–30 MPa for LDPE, but elongation at break for PBAT is often 500–900%, exceeding LDPE's 300–700%. Dart impact resistance, measured under ASTM D1709-22, is generally higher for PBAT film, with typical values of 300–600 g for 20 µm gauge, whereas LDPE of similar gauge often falls between 100–250 g. This property is beneficial for puncture resistance but does not compensate for the loss in stiffness. Elmendorf tear resistance measured under ISO 6383-2:1983 or ASTM D1922-15 may be higher in PBAT, but tear propagation is influenced by orientation and filler; published values for PBAT carrier films range widely from 5 N/mm to 30 N/mm, in contrast to 3–10 N/mm for LDPE. Coefficient of friction for unfilled PBAT is high, often above 0.6 under ISO 8295:1995, which causes blocking and poor bag opening; a combination of 1,000–3,000 ppm erucamide slip and 0.5–2.0 wt% micronised silica antiblock is typically required. These additives must be selected for compatibility with PBAT; some conventional LDPE antiblock masterbatches do not disperse adequately in the polyester matrix. At equal gauge, PBAT film has lower tensile modulus, lower creep resistance, and higher density, so direct substitution requires a redesign of film thickness or bag geometry to meet vertical load and stacking performance.

Seal Bar Temperature, Dwell Time, and Hot-Tack Boundaries

High-cycle T-shirt bag conversion is governed by heat seal initiation, hot tack, and seal strength under automatic wicketting and stacking. PBAT films typically exhibit seal initiation at surface temperatures 80–110 °C, approximately 10–20 °C lower than LDPE initiation temperatures of 100–130 °C, as assessed by heat-seal testing to ASTM F1921-20 and ASTM F88/F88M-21. The lower initiation temperature can reduce seal bar energy consumption, but the hot tack plateau is narrower; above 120 °C, PBAT tends to thin and smear at the seal interface, causing seal failure when the film is immediately tensioned at high cycle speeds. Seal dwell times on rotary bag converters must be increased by 20–40% relative to LDPE, or seal bar temperatures reduced by 10–20 °C, to maintain a seal strength of at least 5 N/15 mm for 20 µm film. The seal strength of PBAT under ASTM F88/F88M-21 reaches 8–15 N/15 mm when the temperature is optimised, comparable to LDPE values of 7–12 N/15 mm, but the thermal conductivity and heat-seal flow characteristics differ; PBAT's higher density reduces cycle speed because more mass must be heated through the seal. In practice, heat-seal bars with non-stick coating are required, and the machine speed may be limited to 100–120 bags/min on servomotor continuous rotary lines, compared with 150–200 bags/min for LDPE. Published data for PBAT at speeds above 120 bags/min on multi-lane wicket bag machines remains limited.

Compliance requirements under EN 13432:2000/AC:2005 and ASTM D6400-23 do not directly validate processing suitability, but they impose constraints on additive selection and film construction. Commercially certified PBAT grades are available with EN 13432 certification under specific thickness and additive combinations; each formulated film must be recertified after changing filler type, slip agent, or processing aid. Biodegradation testing for PBAT film under industrial composting conditions is performed according to ISO 14855-1:2012, with a requirement of at least 90% ultimate biodegradation within 180 days, and disintegration testing under ISO 16929:2021 requires no more than 10% residue on a 2 mm sieve after 12 weeks. The threshold for heavy metals and ecotoxicity is specified in the relevant annex of EN 13432:2000/AC:2005, and the compound must remain within those limits after filler addition. REACH registration under EC 1907/2006 applies to all monomers and additives used in the PBAT compound, but the carrier bag application does not generally require food-contact clearance unless the bag is used for direct food contact. In-line quality assurance for PBAT carrier film must include melt pressure stability, gel-count inspection, online gauge measurement, and periodic seal-strength testing, because the operating window is narrower than LDPE and the failure mode shifts from bubble instability to film-weight compliance and seal rupture.