Sasol PP Copolymer is a heterophasic polypropylene formed by sequential polymerization in a reactor cascade, producing a continuous polypropylene homopolymer matrix with a dispersed ethylene-propylene rubber phase. The product is supplied as pelletized reactor flake for injection moulding, sheet extrusion, thermoforming, and blow moulding applications. Commercial datasheets report melt mass-flow rate under ISO 1133-1 at 230 °C with a 2.16 kg load; general-purpose packaging grades typically fall between 8 g/10 min and 60 g/10 min, while sheet extrusion grades may be specified from 0.5 g/10 min to 3 g/10 min. Density is normally 0.900–0.910 g/cm³ by ISO 1183-1. The heterophasic structure differs from a random propylene-ethylene copolymer because the matrix melting point remains close to 160–165 °C measured by differential scanning calorimetry under ISO 11357-3, whereas random copolymers may show a melting peak from 125 °C to 145 °C. The product is not a homogeneous copolymer; it is a reactor-made polymer alloy whose rubber domains, generally 0.1–2.0 µm in diameter, increase low-temperature ductility without a proportional loss of stiffness.

Why does rubber-phase morphology control low-temperature impact resistance in heterophasic polypropylene?

Low-temperature notched impact resistance depends on the ethylene content of the dispersed phase, the rubber particle size distribution, and the degree of matrix crystallinity. Under ISO 179-1/1eA at −20 °C, general-purpose impact copolymers with ethylene content below 8 wt% often show brittle failure, while grades designed for cold-chain containers may exceed 8 kJ/m²; however, published data for this specific Sasol configuration is limited and should be checked against the supplier’s certificate of analysis. The rubber domains dissipate impact energy through shear yielding and crazing. When rubber content exceeds 25 wt%, flexural modulus measured by ISO 178 can fall below 1000 MPa, reducing suitability for thin-wall rigid packaging. A narrow rubber particle size distribution near 0.5–1.5 µm is generally preferred for toughness; large domains above 3 µm may indicate insufficient dispersion in the reactor or during compounding and can reduce notched impact. The matrix crystallinity, adjusted by nucleating agents, influences the ductile-to-brittle transition. Addition of a nucleating agent at 0.05–0.20 wt% can raise flexural modulus by 5–15% while shifting the ductile-brittle transition upward by a few degrees Celsius.

Injection moulding of the copolymer is commonly performed with a reverse temperature profile from 180 °C at the feed throat to 220–240 °C at the nozzle. A general-purpose three-zone screw with an L/D ratio of 20:1 to 25:1 and a compression ratio of 2.5:1 to 3.5:1 is sufficient. Back pressure is maintained between 5 bar and 10 bar, and screw speed is set at 80–150 min⁻¹. Polypropylene is not hygroscopic; pre-drying is usually unnecessary, but when bags are stored at relative humidity above 60% or transferred from cold storage into a warm production hall, surface condensation should be removed by drying at 70–80 °C for 1–2 h. Mould temperature has a direct effect on skin-layer morphology: raising the mould from 20 °C to 50 °C reduces post-mould shrinkage anisotropy and improves surface gloss in thick-wall automotive parts. In a 1200 kN clamp machine producing a 0.8 mm wall container, sink mark depth was minimized by holding pressure between 50 MPa and 70 MPa and by maintaining a screw cushion of 3–5 mm. Processing temperatures above 250 °C or residence times longer than 5 min can cause chain scission and shift the MFR upward by more than 20%, especially in regrind streams with degraded stabilizer levels. Therefore, the melt temperature should not exceed 240 °C for continuous production runs longer than 8 h unless real-time MFR checks are performed.

In sheet extrusion, the process window is narrower for low-MFR grades than for high-MFR injection moulding grades. A 45 mm single-screw extruder with a 30:1 L/D barrier screw and a flat die lip gap of 2.0 mm requires a die pressure between 80 bar and 120 bar for a 1.5 mm sheet at a line speed of 8–12 m/min. Melt temperature is controlled between 220 °C and 250 °C; above 260 °C, the low-molecular-weight tails in regrind can accumulate on the die lip. Edge trim regrind above 30 wt% has been observed to increase die-lip buildup and require die cleaning every 72 h on a production line. For thermoforming, sheet surface temperature should be uniform within ±2 °C across the forming area; otherwise wall-thickness variation in deep-draw containers can exceed 15%. Moisture from condensation at the hopper, not absorbed water, is the main drying concern. Adding a vented barrel or using a vacuum hopper is unnecessary for virgin pellet but may be helpful when handling high-surface-area regrind stored in humid conditions.

When high ethylene content is required for cold-chain impact

Formulations designed for refrigerated distribution require a balance between the ethylene-propylene rubber content and the rigidity demanded by stackable containers. In instrumented falling-dart tests under ISO 6603-2, a 2 mm plaque from a cold-chain impact copolymer can show puncture energy above 10 J at −20 °C, whereas a homopolymer of equivalent MFR may fail below 2 J. The improvement follows from energy dissipation in the rubber phase, but the rubber is not crosslinked; repeated melt recycling under high shear and temperature can reduce the average rubber domain size, raise MFR, and lower impact strength. After five closed-loop regrind cycles, MFR can increase by 20–50% depending on the stabilizer package and the severity of the heat history. For this reason, the proportion of in-house regrind is usually limited to 15–30 wt% in injection moulding operations and 10–20 wt% in critical automotive interior components, unless the process is validated by tensile and impact testing on the finished part. The ethylene content of the final product is commonly determined by Fourier transform infrared spectroscopy or 13C nuclear magnetic resonance; for impact copolymers, a typical range is 5–15 wt%. Higher ethylene content does not automatically produce higher impact strength if the rubber is coarsely dispersed or if the matrix is over-nucleated.

Primary applications are selected on the basis of MFR and impact level. High-flow grades from 25 g/10 min to 60 g/10 min are used for thin-wall food containers, caps, and closures where short cycle times dominate; medium-flow grades from 8 g/10 min to 25 g/10 min are used for crates, pails, appliance housings, and automotive battery cases; low-flow grades below 3 g/10 min are used for sheet extrusion and thermoforming where melt strength is important. For a given flexural modulus, the impact copolymer allows a downgauging of wall thickness relative to a homopolymer in cold-chain applications, but the lower modulus must be compensated by rib design. In practice, a switch from a homopolymer to an impact copolymer can permit a wall thickness reduction of 10–20% in a transport crate while maintaining identical drop-impact performance at 2 m on a concrete surface; however, published data for this specific Sasol grade is limited and should be confirmed by instrumented drop tests under the customer’s distribution profile.

Relative to a propylene-ethylene random copolymer, Sasol PP Copolymer has a distinctly different property envelope. Random copolymers offer transparency with haze below 20% on 1 mm plaques, low seal initiation temperature, and lower flexural modulus, but their low-temperature impact resistance is comparatively limited. The heterophasic copolymer is generally opaque, with haze above 80% at 1 mm thickness, and is selected where toughness, not clarity, is the primary requirement. Compared with a homopolymer of the same MFR, the heterophasic grade typically shows a reduction in flexural modulus of 20–40% and an increase in notched Charpy impact at 23 °C by a factor of 3–10. Compared with a compounded blend of PP homopolymer and EPDM, the reactor-made heterophasic copolymer can show more consistent rubber dispersion and lower extractables, because the elastomer is formed in situ rather than distributed by melt compounding. However, the reactor-made product can exhibit batch-to-batch variation in MFR and rubber content that must be controlled through the supplier’s polymerization process. Published SPC data for Sasol grades are grade-specific; buyers should request lot-level melt flow and impact data when qualifying a resin for a new tool.

Compared with a compounded thermoplastic olefin produced by melt blending PP homopolymer with EPDM, the reactor-made heterophasic copolymer generally has a lower tendency for rubber agglomeration because the rubber particles are generated during polymerization. Microscopic examination of microtomed sections stained with ruthenium tetroxide often shows a more uniform rubber size distribution in reactor grades than in melt-compounded blends. This can translate into a narrower scatter in notched impact data, with relative standard deviation below 8% in some reactor grades, whereas compounded TPOs may show relative standard deviation above 15% if compounding is not optimized. However, the reactor-made product cannot reach the very high rubber contents above 35 wt% that some compounded TPOs used in automotive bumpers achieve; such high-softness compounds may require secondary blending with additional elastomer in a compounding extruder.

Typical property ranges for general-purpose grades are shown in the following table; Sasol-specific values must be read from the grade datasheet.

The role of stabilizer packages during multiple heat histories

Process stabilizers and antioxidants determine the retention of molecular weight and color during compounding, injection moulding, and end-use service. A standard package for polypropylene impact copolymers includes a hindered phenolic primary antioxidant, a phosphite secondary antioxidant, and a neutralizer such as calcium stearate. The total package may be present at 0.10–0.30 wt%. In multiple-pass extrusion trials on a 40:1 L/D co-rotating twin-screw extruder, a specific mechanical energy input between 0.18 kWh/kg and 0.30 kWh/kg kept melt temperature below 245 °C; above 0.35 kWh/kg, oxidation induction time measured by ISO 11357-6 can decrease by more than 50% after a single pass. The rubber phase is more sensitive to oxidative chain scission than the polypropylene matrix, so impact retention after long-term heat aging is a better indicator of stabilizer performance than MFR stability alone. When the product is to be used in applications requiring continuous service above 80 °C, such as under-hood automotive components, the grade should be qualified by oven aging at 150 °C following ISO 4577 for the required hours. Blending with amine-based additives or strongly basic pigments can deactivate the hindered phenolic antioxidant and should be avoided unless accelerated aging data confirm compatibility. The product should also be protected from prolonged UV exposure unless a UV stabilizer package containing hindered amine light stabilizers and carbon black or titanium dioxide is specified.

Regulatory compliance must be confirmed for each grade and application. Typical polypropylene copolymers can be formulated to meet food-contact requirements under FDA 21 CFR 177.1520 for olefin polymers, with end-use conditions specified by 21 CFR 176.170(c). In the European Union, overall migration into food simulants is evaluated under EU Regulation 10/2011 and must not exceed 10 mg/dm². Automotive interior grades may be tested for volatile organic compounds under VDA 277, with a common target of less than 50 µg C/g, but this is grade-specific. Heavy metal restrictions under RoHS Directive 2011/65/EU Annex II specify a maximum homogeneous material concentration of 1000 mg/kg for lead and 100 mg/kg for cadmium. Under REACH Article 33, suppliers must declare substances of very high concern above 0.1 wt%. These thresholds are not automatic for every grade; they must be verified from the supplier’s regulatory data sheet.