RANPELEN PP Terpolymer SFI-841 is a propylene-ethylene-1-butene random terpolymer supplied as 2–4 mm pellet feedstock for coextruded heat-seal layers in cast polypropylene film, biaxially oriented polypropylene film, and multi-layer flexible packaging. The product is typically characterized by a melt flow rate of 5.0 g/10 min at 230°C/2.16 kg per ISO 1133-1:2022, a density of 0.90 g/cm³ per ISO 1183-1:2019, and a melting endotherm peak near 132°C per ISO 11357-3:2018. Seal initiation on a 30 µm sealant web against a 20 µm biaxially oriented PP substrate, prepared under ASTM F2029-16 and tested per ASTM F88/F88M-21, is typically 110–115°C at 0.5 MPa jaw pressure and 0.5 s dwell. The 1-butene insertion along the propylene chain lowers seal onset relative to a conventional propylene-ethylene random copolymer while retaining sufficient structural stiffness for packaging machine transport.

The grade is positioned for vertical and horizontal form-fill-seal systems where reduced seal jaw temperature widens the operating window against heat-sensitive barrier layers. Published data for the exact seal initiation plateau on a given production line is limited; converter-side verification is required because measured seal initiation shifts with sealant thickness, jaw configuration, and dwell time.

What Limits the Seal Initiation Advantage in Coextruded Cast Film Structures?

The 1-butene-derived comonomer disrupts the regularity of propylene crystallites. Differential scanning calorimetry per ISO 11357-3:2018 generally resolves a broad melting endotherm between 120°C and 145°C and a recrystallization exotherm near 93°C. The depressed crystallite thickness shifts the onset of interfacial fusion below that of a C3/C2 random copolymer; however, the same defect population limits the upper service temperature and seal-food contact stability. Seal strength data generated according to ASTM F88/F88M-21 often show interfacial peel at 105°C, a transition to cohesive peel between 115°C and 125°C, and full cohesive seal strength at 135°C. On rotary VFFS machines running 120 mm wide films at 80 packages/min, the seal jaw setting for SFI-841 can be reduced by 10–15°C compared with a propylene-ethylene random copolymer of equivalent melt flow rate, but this reduction is attenuated when the sealant layer is thinner than 1.5 µm because heat transfer from the jaw is insufficient to melt the full sealant interface.

Optical clarity is governed by cooling rate. On a 40 µm monolayer cast film, haze per ISO 14782 is typically 2.0–2.5% when the melt is quenched on a chill roll maintained at 18–24°C with a water-flow rate of 12–18 L/min per 300 mm die width. Raising the chill roll surface to 28°C increases haze above 4.0% by permitting spherulitic growth. Gloss measured at 60° per ASTM D2457 is typically 90–95 GU on the chill roll side and 85–90 GU on the air side, reflecting the difference in surface replication.

In biaxially oriented PP film, SFI-841 is coextruded as the heat-seal skin layer at 1–2 µm thickness over a PP homopolymer core. The cast web is preheated to 120–135°C before machine-direction stretching at a draw ratio of 4.5–5.0, followed by transverse stretching at 150–160°C with a draw ratio of 8–10. The terpolymer skin remains heat-sealable after orientation because 1-butene suppresses crystallization; however, insufficient preheat or excessive transverse draw can create surface streaks. Measured gloss at 20° per ASTM D2457 can fall below 70 GU when these defects occur.

Dynamic oscillatory rheometry at 220°C per ISO 6721-10:2015 typically records a complex viscosity at 1 rad/s of approximately 3500 Pa·s and a shear-thinning index between 100 s⁻¹ and 1000 s⁻¹ of 0.30–0.40. The pseudoplasticity allows adequate filling of thin die lips but limits the draw ratio in high-speed cast film. Melt elongation at 230°C is lower than that of high-melt-strength PP grades, so coextrusion with a high-melt-strength core is advised when line speed exceeds 180 m/min.

Processing on cast film lines with a 75 mm or 90 mm single-screw extruder having 30:1 L/D and a barrier screw with Maddock mixing tip uses barrel zone settings from 210°C at the feed to 240°C at the adapter, and a die temperature of 240°C. Melt pressure at the screen changer with a 200/400/200 µm screen pack is typically 25–35 MPa at 80 rpm depending on die width and lip gap. The low melt strength of SFI-841 relative to a high-melt-strength PP makes the web susceptible to draw resonance above 150 m/min when the die lip gap is below 0.6 mm and melt temperature is below 225°C. Maintaining a melt temperature of 235–245°C lowers head pressure but reduces melt strength further, so edge pinning with electrostatic pins at 6–8 kV DC and vacuum edge boxes is standard in production. No pre-drying is required when pellet surface moisture is below 100 ppm; however, humid silo storage can cause intermittent screw slip and melt pressure variation of ±3 MPa.

Slip and anti-block concentrates are typically added at 1–2 wt% of a masterbatch containing 5% erucamide and 10% synthetic silica, yielding a letdown slip concentration of 500–1000 ppm and anti-block concentration of 1000–2000 ppm. Over-addition of slip above 2000 ppm reduces seal strength and increases plate-out on casting rolls, which in turn creates transfer surface defects and lowers seal continuity.

When Relative Humidity Exceeds 60% During Storage

Although the base polypropylene is hydrophobic, additive particles and pellet surfaces can adsorb moisture. In tropical warehouses with relative humidity above 60%, surface moisture can exceed 500 ppm. On grooved feed extruders this produces screw slippage, feed instability, and melt pressure oscillation of ±3 MPa. Pre-drying in a desiccant hopper at 80°C for 2 h reduces surface moisture below 100 ppm and stabilizes gravimetric throughput. Drying should not exceed 4 h at 80°C, because prolonged heating can volatilize low-molecular-weight slip agents and shift the film coefficient of friction after 14 days of additive migration. The coefficient of friction of the sealant side measured according to ISO 8295 at 23°C is typically 0.20–0.30 after 14 days; pre-drying beyond 6 h can raise the kinetic coefficient above 0.35 because less erucamide remains available to bloom to the surface.

For food-contact structures, the converter must assess compliance of the final film under FDA 21 CFR 177.1520 for olefin polymers and EU Regulation (EU) No 10/2011 Annex I. SFI-841 is normally used as a thin sealant layer over PP homopolymer or copolymer core; the food-contact layer must be covered by a specific migration or overall migration test if fatty food simulants are involved. Overall migration testing according to EN 1186-1:2002 and EU 10/2011 Annex V with simulant D2 or 95% ethanol is generally required for high-fat contact. The product is supplied with a regulatory statement that no SVHCs exceed 0.1% by mass per REACH Article 33; RoHS screening is not typically applicable to polyolefin films, but converters must verify downstream printing and lamination inputs separately.

Comparative Thermal and Seal Property Matrix Against C3/C2 Random Copolymer and C4/C6 Plastomer Blends

The product differs from standard propylene-ethylene random copolymers through 1-butene insertion, which lowers seal initiation and broadens hot-tack while reducing melting point and flexural modulus. Relative to C4/C6 plastomer-modified sealant formulations, SFI-841 offers higher stiffness and lower extractables, but lower sub-ambient impact resistance. Table 1 lists typical announced values; these are not lot-specific release limits.

Hot-tack response is a critical differentiator in high-speed packaging. Table 2 gives typical heat-seal strength development as a function of jaw temperature for a 25 µm monolayer film sealed at 0.5 MPa for 0.5 s and pulled per ASTM F88/F88M-21. The transition from interfacial to cohesive failure occurs between 115°C and 125°C.

At the converter level, SFI-841 is often selected over a C3/C2 random copolymer when the packaging line uses heat-sensitive substrates such as metallized BOPP or EVOH barrier. The lower seal threshold reduces thermal load on the barrier layer, but it also narrows the temperature gap between hot-tack and seal integrity. If the seal jaw temperature is set below 105°C, seal strength remains below 2.0 N/15 mm, and leakage may occur in drop tests according to ASTM D5276-19. Therefore the practical operating window is 115–135°C with a dwell time of 0.5 s or greater.

Operational boundaries for SFI-841 are defined by thermal stability and additive compatibility. The resin is not intended for retort pouches or steam sterilization above 121°C because the sealant layer softens and seal creep can occur above 100°C. It should not be processed at melt temperatures above 260°C for residence times longer than 15 min because chain scission increases MFR and can shift seal initiation upward by 3–5°C. The resin is not an alloying grade; addition of more than 10 wt% of C4/C6 plastomer depresses stiffness below the range required for stand-up pouch structures and increases extractables. Additive masterbatches containing amine-based slip boosters are not recommended due to plate-out on cast rolls and loss of seal strength.