PTTGC LDPE Grades: Thai Resin for Film and Packaging Converters

Low-density polyethylene (LDPE) supplied by PTT Global Chemical Public Company Limited is a high-pressure, free-radical polymerised polyolefin with a density range of 0.917–0.930 g/cm³ determined by ASTM D1505-18 or ISO 1183-1:2019. The resin is a branched, partially crystalline thermoplastic processed by Thai film and packaging converters on blown film, cast film, extrusion coating, and laminating lines. Melt flow rate, measured at 190 °C under 2.16 kg load according to ISO 1133-1:2022, separates film grades in the approximate range 0.2–4.0 g/10 min from extrusion coating grades in the range 4.0–10.0 g/10 min. Long-chain branching and broad molecular weight distribution contribute shear thinning and extensional melt strength, which are prerequisites for bubble stability and low neck-in. Published grade-specific data should be obtained from the producer’s certificate of analysis because additive packages and molecular architecture differ between delivery formats.

What Molecular Parameters Control LDPE Film Properties?

Density controls crystallinity and therefore the balance between stiffness, barrier, and optics. A shift from 0.919 g/cm³ to 0.925 g/cm³ raises crystalline fraction and increases 1% secant modulus, measured by ASTM D882-12, while reducing clarity and increasing water vapour barrier measured by ASTM F1249-20 at 38 °C and 90 % RH. Melt flow rate is an inverse indication of molecular weight: film grades with lower melt index produce higher tensile strength and puncture resistance but require elevated head pressure and screw torque. Flow index alone does not capture long-chain branching. Long-chain branching in LDPE results from intermolecular chain transfer during high-pressure polymerisation; it raises extensional viscosity and strain hardening, which stabilise the blown film bubble. Conventional gel permeation chromatography is not fully accurate for LDPE because branched molecules do not obey the same hydrodynamic volume relationship as linear polyethylene.

The processing window narrows when converters blend LDPE with linear low-density polyethylene. At LDPE addition below 15 wt%, bubble stability may remain inadequate on high-stalk lines; above 30 wt%, dart impact and tear resistance often show a nonlinear decrease. This is a property cliff-edge rather than a linear dilution because the branched fraction alters crystal tie-molecule populations and tear propagation paths. Shear viscosity at 190 °C and 100 s⁻¹ typically falls between 300 Pa·s and 800 Pa·s for film grades, but extensional viscosity at strain rates near 1–10 s⁻¹ dominates melt strength. Capillary rheometry according to ISO 11443:2021 provides shear viscosity and entrance pressure loss; extensional behaviour is measured by Rheotens or filament stretching.

Blown Film Die Conditions and Bubble Geometry

Blown film conversion of PTTGC LDPE grades is typically carried out on single-screw extruders with screw L/D 24:1–30:1 and barrier sections or Maddock mixing elements. Die gaps for LDPE are commonly set between 0.8 mm and 1.8 mm; wider gaps improve film optics and melt strength but raise residence time and oxidative risk. Spiral mandrel dies are preferred for monolayer film because they distribute melt with lower stagnation risk than spider dies. Melt temperatures at the die are maintained at 160–200 °C. Blow-up ratios of 2.0:1–3.0:1 are normal; high-stalk configurations may use 3.5:1 to balance machine-direction and transverse-direction tensile strength. Frost line height is adjusted to 4–10 die diameters. Raising frost line height improves optical clarity but increases bubble instability under plant air currents.

Bubble instability is influenced by extensional viscosity, cooling air turbulence, and die temperature uniformity. Melt pressure at the die is typically 150–350 bar depending on melt index, die gap, and output. Temperature differences across the die lip greater than 5 °C generate gauge variation that is amplified by the film collapsing frame. For packaging converters running 50 µm monolayer film, thickness tolerance is controlled to ±10 % or tighter per ISO 4593 or ASTM D6988-21. Dart drop impact measured by ASTM D1709-15a and Elmendorf tear measured by ASTM D1922-15 are sensitive to film gauge uniformity and frost line position.

Process conflict arises between optical quality and impact strength. Higher frost lines increase gloss and reduce haze but alter orientation gradients and can lower dart impact; the magnitude of this effect is line-dependent and not linear. On a production line, changing frost line height requires re-verification of gauge profile and seal response. Melt temperature reduction from 190 °C to 170 °C improves bubble stability but raises viscosity and screw torque; without an increase in back pressure, the screw may become starved. This coupling between temperature, output, and bubble geometry is a recurrent operational bottleneck on Thai packaging lines during seasonal humidity changes.

In extrusion coating, the resin is processed at melt temperatures of 280–320 °C to promote adhesion to paper, paperboard, aluminium foil, or metallised films. The die gap is typically 0.5–1.0 mm, and coat weights range from 8 g/m² to 40 g/m². At these temperatures, molecular weight degradation can occur if residence time is excessive; converters monitor melt pressure and gel formation at the die lip. Neck-in, the reduction in web width between die exit and substrate contact, is lower for LDPE than for most linear polyolefins because long-chain branching increases melt strength. Draw-down behaviour is limited by melt fracture at high line speeds; die exit velocities above the critical shear rate produce sharkskin. Adhesion to aluminium foil is assessed by ASTM D903-17 for peel resistance, while package seal strength is measured by ASTM F88/F88M-21. Corona discharge and ozone treatment are applied to raise surface energy from 30–32 mN/m to 38–42 mN/m before lamination.

Adhesion to aluminium foil is sensitive to melt temperature and air gap. At melt temperature below 280 °C, oxidation at the melt surface may be insufficient for strong polar adhesion to aluminium, and peel strength can fall below the practical minimum of 2.0 N/15 mm. The air gap between die exit and nip should be maintained at 100–200 mm; longer gaps raise neck-in and oxidation but can reduce adhesion if the web speed draws the melt beyond its heat loss budget. For liquid packaging board, coat weight uniformity of ±10 % is controlled by die bolt adjustment and online beta/gamma gauge measurement.

When Cast Film Chill Roll Temperatures Fall Below the Optical Quench Threshold

Cast film lines running PTTGC LDPE grades differ from blown film in cooling rate and orientation. The melt is extruded through a flat die, usually with a die gap 0.5–0.8 mm, onto a polished chill roll maintained at 15–25 °C. Rapid quench suppresses spherulite growth and produces high-gloss film with haze below 5 % at thicknesses under 30 µm, measured by ASTM D1003-13. If chill roll temperature rises above 30 °C, LDPE leaves the roll with excess crystallinity and internal haze increases, even if water bath temperature is unchanged. Contact uniformity depends on air knife pressure and electrostatic pinning; insufficient pinning permits air entrapment that reduces quench rate and creates gauge bands.

Machine-direction orientation in cast film is lower than in blown film, so tensile properties are more isotropic. Draw resonance occurs when draw ratio exceeds the melt’s ability to resist extensional thinning. The critical draw ratio for LDPE is higher than for linear low-density polyethylene because branched architecture provides strain hardening. Line speed and die-to-roll gap must be controlled to avoid edge beads and high-telescope roll defects. Thickness profile is measured by ASTM D6988-21 or an online capacitance gauge; edge trim recycled at up to 20 wt% is common, but higher recycle fractions increase gels and optical defects.

Heat seal response of LDPE packaging films is governed by the amorphous phase, density, and surface treatment. Seal initiation temperature is commonly 95–115 °C for film grades without excessive high-slip or anti-block loading, and hot tack is measured by ASTM F1921-20 across a temperature window of 110–130 °C. Seal strength is reported by ASTM F88/F88M-21 after a dwell time of 0.5–1.0 s and pressure of 200–400 kPa. Contamination by medium-density polyethylene or uneven corona treatment raises seal initiation by several degrees and narrows the hot tack plateau. For laminates, the LDPE layer serves as both sealant and adhesive; sealant performance is undermined by excessive film age because migratory slip additives bloom to the surface and reduce interfacial seal strength.

Converters running high-speed vertical form-fill-seal machines require stable hot tack above 1.0 N/25 mm to prevent package opening when product drop loads the seal before cooling. This threshold depends on film thickness and seal bar profile. Published data for this specific configuration is limited, so line trials remain necessary to verify PTTGC LDPE grade suitability under production conditions.

Food Contact and Migration Compliance Matrix

Regulatory status for PTTGC LDPE grades used in food contact depends on the polymerisation additives and the converter’s final article. The base polyethylene falls within olefin polymers permitted under FDA 21 CFR 177.1520, with conditions of use assigned according to 21 CFR 176.170. For European converters, compliance with Regulation (EU) No 10/2011 requires overall migration below 10 mg/dm² of food contact surface and specific migration limits for authorised additives. Documentation must be maintained under Regulation (EC) No 2023/2006 on good manufacturing practice for food contact materials. Substances of very high concern under REACH Regulation (EC) No 1907/2006 must be below 0.1 % w/w in the article.

For halal and kosher packaging lines, certification of PTTGC LDPE grades is supplied at the resin stage but is not a substitute for converter site certification. Migration testing is performed under the EN 1186 series for overall migration and EN 13130 for specific migration. The resin supplier’s food contact statement covers only the base polymer; inks, coatings, and adhesives used downstream must be assessed separately.

When sustained melt temperature exceeds 300 °C, oxidative degradation of PTTGC LDPE proceeds by chain scission and crosslinking; extrusion coating lines should limit residence time and use inert gas in the feed throat if start-up exceeds 15 min. The polymer is not suitable for sustained exposure to strong oxidising acids, chlorinated solvents, or aromatic hydrocarbons because swelling and environmental stress cracking can occur. Continuous service temperature is generally below 80 °C, with short thermal excursions up to 95 °C for empty package storage. Film stored under ultraviolet light without UV stabiliser embrittles; outdoor exposure beyond 6 months requires carbon black or hindered amine stabiliser addition. Pre-drying is not normally required at ambient humidity below 60 % RH; when stored in cold rooms, condensation on pellets can form, so hopper dryers at 40–50 °C for 2 h may be used before processing.