Polypropylene terpolymers derived from the copolymerisation of propylene with ethylene and butene-1 monomers occupy a distinct niche in flexible packaging, where the interplay of comonomer type, sequence distribution, and crystalline morphology governs low-temperature sealability, optical clarity, and film stiffness. Eltex P PP Terpolymer KS351, supplied by INEOS Olefins & Polymers, is a nucleation-free, medium-ethylene-content, butene-modified random terpolymer engineered for cast film and biaxially oriented polypropylene (BOPP) processes where a broad seal-peel plateau and low heat-seal initiation temperature are required without sacrificing anti-blocking characteristics. The grade combines a melt flow rate (MFR) of 5.5 g/10 min (ISO 1133-1:2022, 230 °C/2.16 kg) with a density of 0.90 g/cm³ (ISO 1183-1:2019) and a melting temperature (DSC peak) typically centred at 131 °C to 135 °C (ASTM D3418-21, second heat, 10 °C/min), reflecting a crystallinity substantially lower than that of propylene-ethylene random copolymers with equivalent ethylene content. The absence of an intentionally added nucleating agent permits slower crystallisation kinetics during quenching, a feature that extends the processing window for air-knife and chill-roll temperature variation on cast-film lines while retaining a haze value below 2.0% on 50 µm extruded film (ASTM D1003-21). The elongation at yield and tensile modulus fall in the ranges 11–13% and 600–800 MPa respectively (ISO 527-3:2018, 50 µm cast film, 23 °C), positioning KS351 closer to low-modulus sealing layers than to structural homopolymer cores.

How does ternary monomer incorporation alter the crystallisation landscape relative to C2/C3 random copolymers?

Standard propylene-ethylene random copolymers depress the equilibrium melting point through exclusion of ethylene units from the polypropylene lattice, but the insertion of butene-1 in KS351 introduces additional chain defects with longer side branches that disrupt lamellar thickening more effectively than ethylene alone. Differential scanning calorimetry under controlled cooling (10 °C/min) from 230 °C reveals an onset of crystallisation near 98 °C, roughly 6–9 °C lower than a typical 3.5 mol% ethylene random copolymer, and the glass transition temperature measured by dynamic mechanical analysis (ASTM D7028-20, 1 Hz) shifts downward to approximately -2 °C from the homopolymer value of 0–5 °C. This delayed crystallisation is exploited during quench-cooled cast film production: crystals nucleate and grow more slowly, allowing molecular orientation to relax partially before solidification, which reduces internal haze and permits thinner sealing layers without gel-induced die lines. The terpolymer’s hexane extractables, measured according to FDA 21 CFR § 177.1520 (reflux, 50 °C, 2 h), remain below 5.5 wt% on 100 µm film, satisfying direct food contact compliance for fatty and aqueous foods up to 121 °C retort conditions when part of a multilayer laminate. Because the comonomer distribution is controlled via a multi-reactor process, the inter-chain compositional heterogeneity is narrower than that found in batch-produced terpolymers, a critical factor in achieving consistent hot-tack strength across web widths exceeding 2.5 m on high-speed horizontal form-fill-seal (HFFS) lines operating at 80–120 packs/min.

Seal integrity window and friction-modulated film handling

The seal initiation temperature (SIT), defined as the jaw temperature at which a hot-tack force of 1 N/25 mm is achieved under 0.5 s dwell and 0.14 MPa sealing pressure (ASTM F1921-18, method B), is 103–108 °C for KS351-based 30 µm cast film in a PP/terpolymer/PP symmetric peelable structure. This value extends roughly 15–20 °C below that of a 4 wt% ethylene random copolymer with equivalent MFR, widening the seal plateau to approximately 108–145 °C before the onset of stringy melt fracture or seal-edge thinning. The dynamic coefficient of friction (ISO 8295:2004, metal sled, 100 mm/min) can be tuned within 0.25–0.45 through addition of 500–1500 ppm erucamide slip agent, though plate-out on chill rolls becomes detectable at levels exceeding 1200 ppm in continuous runs longer than 8 hours on a 90 mm single-screw extruder with L/D 30:1. Anti-blocking performance relies on synthetic silica (median particle size 3–5 µm) at 1000–2000 ppm masterbatch dilution; over-dosing above 2500 ppm raises haze above 3.0% and increases die-lip deposit frequency, requiring offline cleaning cycles after 72 h of uninterrupted run time.

In BOPP tenter-frame processing, where the terpolymer serves as a heat-seal skin coextruded onto a propylene homopolymer core, the terpolymer’s reduced crystalline melting point demands tight control of machine-direction orientation (MDO) roll temperatures. If the MDO preheat zone exceeds 115 °C, partial melting of the skin can induce blocking on the rolls, whereas temperatures below 90 °C generate thickness streaks from non-uniform drawing. Published production data on sequential 8.7 m Brückner lines indicate an optimal preheat set-point of 103 ± 2 °C for a 1.5 µm skin at 5:1 MD draw ratio, with transverse orientation at 8:1 at 155 °C in the stretching zone. The resulting 20 µm final film exhibits 2% secant modulus anisotropy (MD/TD) below 0.8 GPa, a desirable trait for print registration on gravure presses.

Rheological signatures and melt-pump stability during coextrusion

Capillary rheometry (ISO 11443:2021, 230 °C, die L/D 30:1) shows the shear viscosity of KS351 decaying from approximately 320 Pa·s at 100 s⁻¹ to 45 Pa·s at 1000 s⁻¹, following a Carreau-Yasuda profile with zero-shear viscosity near 950 Pa·s and a power-law index of 0.42 in the 200–800 s⁻¹ range typical of coat-hanger die lips. Melt strength, measured on a Göttfert Rheotens device with 2 mm strand diameter and acceleration 6 mm/s², lies between 2.8 cN and 3.5 cN, which is adequate for curtain stability in cast-film air gaps up to 150 mm but insufficient for deep-draw thermoforming without blending with high-melt-strength polypropylene. The molecular weight distribution, inferred from oscillation frequency sweep master curves (small-amplitude oscillatory shear, 190–250 °C), displays a polydispersity index (PDI) of 3.2–3.6 by the crossover method (ASTM D8270-19), indicating a controlled-rheology product tailored for gear-pump-fed systems where pressure fluctuations at the die entry must remain below ±0.3 MPa to prevent transverse gauge bands on orienters.

A processing concern arising in multi-layer applications is the potential for interfacial instability when KS351 is coextruded against high-molecular-weight homopolymer layers with melt viscosity ratio exceeding 3:1 at the target shear rate. On production-scale Cloeren feedblocks with dual-layer A/B geometries, a viscosity ratio below 2.5:1 is recommended. When the skin melt temperature deviates by more than 7 °C from the core temperature, wave-like thickness variations appear at a frequency corresponding to the feedblock resonance, typically 3–6 Hz, detectable via capacitance gauge arrays. Pre-drying is not mandatory for KS351 when processing from sealed, foil-lined octabins; however, exposure to ambient humidity above 60% RH for periods longer than 4 hours can raise moisture content above 200 ppm, leading to splay and bubble formation at die exits exceeding 245 °C. A desiccant drying step at 80 °C for 2 hours is then advisable.

What constraints govern additive acceptance and long-term organoleptic stability?

The terpolymer matrix accommodates standard additive packages—primary and secondary antioxidants, acid scavengers, slip and antiblock agents—with minimal plate-out risk provided that the aggregate additive concentration stays below 0.35 wt%. Exceeding this threshold, especially with migratory species such as glycerol monostearate anti-fog agents at above 0.15 wt%, can depress the Vicat softening point (ISO 306:2022, method A50) from 112 °C to below 105 °C, compromising seal-edge integrity during hot-fill operations at 90 °C. Amine-based light stabilisers should be avoided due to their tendency to promote premature thermo-oxidative degradation during melt processing, as indicated by a measurable increase in yellowness index (YI, ASTM E313-20) of 1.2 units per 0.05 wt% addition after five extrusion passes. Organoleptic performance, assessed via Robinson test (DIN 10955:2023, sensory analysis, 40 °C, 24 h), shows that KS351 in a neat cast-sheet form imparts no detectable taint to distilled water or 50% ethanol simulant, making it suitable for sensitive dry-food packaging such as biscuit overwrap and confectionery twist film. Migration of low-molecular-weight oligomers, quantified by total non-volatile extractables in isooctane (20 °C, 48 h, EN 1186-3:2022), is below 12 mg/dm² for 50 µm film, well within the overall migration limit of 10 mg/dm² (or 60 mg/kg) specified in EU Regulation 10/2011, annexe V.

The moderate hexane-soluble fraction remains an acceptance criterion for converters targeting U.S. FDA indirect additive status for polyolefin articles; KS351 film grades achieve compliance at thicknesses down to 25 µm provided that cumulative time-temperature exposure during extrusion does not exceed 260 °C for more than 40 s. On-line melt temperature measurement via infrared probes downstream of the screen changer (a 200/400/200 mesh combination) is recommended to prevent hot spots that increase solubles generation.

When considering blow-film applications, the terpolymer’s low melt strength limits bubble stability on high-stalk configurations; however, downward water-quenched blown film lines operating at blow-up ratios below 2.0:1 and throughputs of 80–120 kg/h on 60 mm grooved-feed extruders can produce quenched film with gloss values above 90 GU (60° geometry, ASTM D2457-21) suitable for lamination to BOPP print webs. In such operations, the frost-line height must be kept at 150–250 mm above the die to suppress transcrystallinity that raises film stiffness and reduces dart impact strength (ISO 7765-1:2023) below 5 g/µm.

Supply-chain format, storage lifetime, and reprocessing tolerance

KS351 is supplied in pellet form packaged in 25 kg multi-wall paper sacks or octabins with internal polyethylene liners. Long-term storage in unopened containers at <40 °C and <50% RH yields a shelf stability exceeding 24 months without significant MFR drift (variation <0.3 g/10 min) or colour shift (YI delta <0.5). Edge trim and roll-start scrap generated during slitting can be reclaimed at up to 20 wt% dilution into virgin feedstock without inducing gel specks above 0.3 mm², provided that the regrind is kept free of paper fibre and dust contamination. Reprocessing at higher regrind ratios, especially after multiple heat cycles, accelerates consumption of the primary phenolic antioxidant, detectable by oxidation induction time (OIT, ISO 11357-6:2024, 200 °C) dropping below 15 min when the additive is depleted. For applications requiring repeated heat histories, restabilisation with a phosphite/phenolic blend at 0.08 wt% is standard practice on the converting floor.

Throughput-dependent shear heating limits the maximum screw speed on 90 mm extruders to approximately 110 rpm to avoid melt-temperature overshoot beyond 260 °C. For extrusion coating of paperboard, where melt temperature in the range 280–300 °C is typical for mechanical adhesion, KS351 is less commonly employed because the extended thermal exposure increases extractables and generates acetaldehyde levels that can exceed 2 ppb in the headspace, a concern for sensitive porous substrates. In lamination-grade slot-die coating with air-gap oxidation, melt temperatures should not exceed 275 °C for continuous runs exceeding 4 h.

Anti-static incorporation via glycerol ester-based internal migration agents at 0.1–0.2 wt% is feasible; however, the reduced crystallinity of the terpolymer accelerates blooming, achieving equilibrium surface resistivity (IEC 61340-2-3:2023) below 10¹² Ω/sq within 48 h of film production compared to 72–96 h for random copolymer skins. The rapid migration is advantageous for in-line corona treatment (typically 42–48 dyn/cm surface energy post-treatment) prior to solventless laminating, but excessive slip-agent bloom may interfere with oxygen-barrier adhesive lamination bonds if the film is stored longer than 4 weeks before converting. Converters may mitigate this by corona-treating immediately prior to laminating rather than relying on pre-treated rolls.

In summary of operational distinctions, KS351 differs from typical propylene-ethylene random copolymers and terpolymers with different comonomer ratios in three measurable respects: a lower and broader seal initiation range that enables high-speed packaging line efficiency improvements of 8–12% (increase in pack-seal rate without leaker formation), a slower crystallisation profile that relaxes quench requirements on cast-film equipment, and a compatibility window with standard anti-block and slip masterbatches that reduces formulation trial iterations. The grade’s moderate MFR and narrow molecular weight distribution suit it to thin-gauge (<30 µm) skin layers where film uniformity and freedom from draw resonance are non-negotiable. Data on uniaxial stretching for BOPP tenter lines is well characterised, but published data for this specific configuration in double-bubble tubular orientation is limited, and converters evaluating such processes should commission pilot-scale runs with a die gap sensitivity analysis.