MARPOL PP Homopolymer H 400 is an unfilled polypropylene homopolymer supplied for injection moulding of rigid packaging, caps, closures, and thin-wall houseware components. The commercial grade designation H 400 is a supplier identifier; it is not an ISO 1133-1:2022 melt flow rate code, and lot-specific melt viscosity must be verified against the certificate of analysis. The material is specified around a controlled molar mass distribution, a density of 0.900–0.910 g/cm³ under ISO 1183-1:2019, and a crystallinity-driven stiffness profile that separates it from random copolymer and impact copolymer polypropylene products. Because the repeating unit is exclusively propylene, the resin exhibits higher dimensional stability under load at elevated temperatures but lower multi-axial impact toughness at sub-zero conditions than grades containing ethylene comonomer.

The extrusion and compounding history is controlled to limit chain branching and gel formation. Supplier documentation for the H 400 grade ordinarily lists melt volume flow rate, tensile yield stress, flexural modulus, Charpy notched impact strength, and heat deflection temperature as the primary release criteria. For a 400-series unfilled homopolymer injection grade, the typical melt flow rate measured at 230 °C under 2.16 kg piston load falls in the 10–14 g/10 min range; however, published data for this specific H 400 configuration is limited, and the certificate of analysis remains the controlling document.

What Melt Flow and Mechanical Boundaries Define H 400?

Melt flow rate is determined according to ISO 1133-1:2022, Method A, and is used as an indirect indicator of number-average molar mass and shear-thinning response during cavity filling. In thin-wall closure moulding, a melt flow rate below 8 g/10 min usually increases injection pressure demand and can produce short shots in multicavity hot-runner tooling; a melt flow rate above 18 g/10 min may reduce impact strength and increase the risk of hinge whitening in integral closures. The H 400-controlled envelope is therefore placed between these operational boundaries in distributor documentation. Tensile yield stress is evaluated on ISO 527-2:2012 type 1A specimens at 50 mm/min. Commercial unfilled homopolymer PP of this class typically shows yield stress between 33 MPa and 37 MPa, with strain at yield near 8–10%. Flexural modulus tested under ISO 178:2019 at 2 mm/min generally ranges from 1,400 MPa to 1,700 MPa. These values determine ejection timing because the modulus falls rapidly above the crystallization plateau but remains high enough to allow demoulding of undercut closure features only when mould geometry has adequate draft angles.

The table below is a representative property envelope for unfilled 400-series homopolymer injection moulding grades, not a guaranteed lot-specific certificate.

Charpy notched impact at 23 °C is commonly reported near 2.5–4.0 kJ/m² under ISO 179-1/1eA, while the same test at 0 °C typically falls to 1.5–2.0 kJ/m²; this is the property cliff-edge that limits H 400 use in freezer-grade packaging. The heat deflection temperature under 0.45 MPa is approximately 95–105 °C, which supports hot-fill and warm-close applications but does not justify continuous service above the Vicat softening range.

Controlling Melt Temperature, Clamp Force, and Shrinkage Anisotropy

On reciprocating-screw machines with barrier or general-purpose screws of 20:1 to 24:1 L/D ratio, melt temperature should be held within 220–250 °C as measured by a nozzle pyrometer, and mould temperature should be maintained between 20 °C and 50 °C for cycle-time control. For thin-wall containers with nominal wall stock of 1.0–1.5 mm, injection velocity is usually set to 150–250 mm/s, and hold pressure is optimised between 60% and 80% of peak injection pressure. The narrow semicrystalline processing window of homopolymer PP requires melt residence time below 30 min at melt temperatures above 250 °C; longer residence produces chain scission, yellowing, and loss of notched impact strength. Clamp force planning for H 400 follows a projected-area demand of approximately 3.5–5.0 kN/cm². A 64-cavity closure mould with a projected shot area of 1,200 cm² therefore requires a machine in the 4,200–6,000 kN clamp force class, although hot-runner balancing and gate freeze-off modify the actual requirement.

Shrinkage of unfilled homopolymer PP is anisotropic. Longitudinal and transverse shrinkage after 24 h under ISO 294-4:2018 typically lies between 1.2% and 1.8% for injection-moulded plaques. The exact value depends on melt temperature, mould temperature, holding pressure, and wall thickness. Because H 400 does not include an elastomer phase, post-mould warpage is driven primarily by crystallinity gradients across the part thickness; non-uniform cooling in corners and gate regions produces visible sink marks and dimensional drift. The use of conformal cooling channels with 3–5 mm diameter and coolant Reynolds numbers above 10,000 is documented in production environments to reduce warpage in polypropylene closure lids, though published data for this specific H 400 configuration is limited.

In high-cavitation hot-runner moulding of caps and closures, valve-gate sequencing and hot-runner manifold temperatures in the 230–260 °C range are used to prevent premature solidification in the sub-runner. The homopolymer grade has a narrow plateau between melt viscosity stability and thermal degradation; if the hot-runner temperature exceeds 270 °C, residence time should be reduced below 10 min. Integral hinge designs rely on high shear orientation during fill and immediate flexing after demoulding to develop crystalline orientation across the hinge line. Failure modes observed on production-scale closure lines include hinge whitening at hinge thickness below 0.25 mm, delamination from weld lines when melt temperature is below 220 °C, and cap skirt cracking when mould opening is delayed and part temperature falls below 60 °C before hinge flexing.

When H 400 Replaces Random Copolymer or Filled PP in Rigid Packaging

The principal design consequence of selecting H 400 over a random copolymer is the increase in flexural modulus and heat deflection temperature at the expense of low-temperature impact. Random copolymer PP with 2–4% ethylene content typically shows a notched Charpy impact at 0 °C of 4–6 kJ/m², whereas the homopolymer H 400 class falls closer to 1.5–2.0 kJ/m². This difference makes random copolymer the safer choice for freezer containers and injection blow-moulded bottles, but H 400 is preferred when stacking strength, hot-fill resistance, and chemical stress crack resistance to oils and surfactants are dominant. The absence of ethylene in the backbone also reduces extractable content, which is relevant for pharmaceutical closures and food-contact packaging.

Compared with talc-filled PP homopolymer compounds, H 400 has lower density, lower stiffness, and higher mould shrinkage. Talc-filled grades at 20 wt% talc typically raise flexural modulus to 2,500–3,500 MPa and reduce shrinkage to 0.7–1.0%, but they also lower weld-line strength and increase abrasion in screw and barrel. H 400 does not generate the same screw and check-ring wear, and it does not require the same drying vigilance as hygroscopic fillers under high humidity. However, when dimensional stability in large flat automotive panels or appliance bases is required, the unfilled H 400 grade is not a direct substitute because its shrinkage anisotropy and lower modulus cannot match mineral-filled performance.

These comparative values are drawn from commercial datasheets for injection-moulding grades and are not substitute values for the H 400 certificate of analysis. The structural difference is most visible in weld-line strength: unfilled homopolymer can retain a higher proportion of the base tensile strength across a weld line than talc-filled material because the mineral platelets do not reorient across the meeting front.

Regulatory positioning of H 400 depends on the additive package and polymerization aids selected by the producer. When ordered for food-contact applications, the grade is commonly supported by a declaration of compliance to EU No 10/2011, including overall migration testing under OM2 or OM7 conditions depending on intended contact, and by FDA 21 CFR 177.1520 for olefin polymers. Pharmacopoeial grade material designed for packaging or closures may be tested under Ph. Eur. 3.1.3 or USP <661>. The product is not a hazardous mixture under REACH Regulation (EC) No 1907/2006, but each lot must be assessed for substances of very high concern present as impurities. Heavy metal and halogen content can be kept within RoHS Directive 2011/65/EU limits for electrical and electronic equipment components, though unfilled polypropylene is not the typical source of regulated brominated flame retardants. Compliance statements should be obtained from the supplier for each specific production batch because additive packages vary by application.

Thermal Degradation Pathways and Additive Compatibility Boundaries in H 400

Before melt processing, the stabilizer package in H 400 must be considered because unstabilized polypropylene homopolymer undergoes auto-oxidative chain scission with an activation energy of approximately 100–120 kJ/mol in the melt. H 400 is supplied with a stabilizer package typically containing a phenolic primary antioxidant and a phosphite secondary antioxidant, but the exact composition is supplier-controlled. Repeated processing or use of regrind above 30 wt% may shift the oxidation induction time downward and produce yellowing in transparent or lightly coloured parts. Oxidation induction time tested by differential scanning calorimetry under ISO 11357-6:2018 is a useful lot-to-lot consistency check; a drop below 10 min at 200 °C in oxygen is often treated as a thermal stabilizer depletion indicator in production quality control.

H 400 is incompatible with copper-based heat stabilizers and certain amine-based additives that can accelerate thermo-oxidative degradation or cause acid-catalysed hydrolysis in hydrolytically sensitive pigments. Avoid combination with unsaturated oils, strong oxidising agents, and high levels of low-molecular-weight hydrocarbon processing aids because these can plasticize the matrix and lower heat deflection temperature. Pre-drying is not required when packaging remains sealed and ambient relative humidity is below 60%. If surface condensation occurs during cold warehouse storage, a desiccant dryer at 80 °C for 2–4 h with a dew point below −40 °C is sufficient; moisture is primarily surface-bound because the homopolymer backbone is hydrophobic.

Outside packaging, H 400 is specified for injection-moulded laboratory consumables, pipette tips, and diagnostic device housings where chemical resistance to dilute acids, bases, and aqueous buffers is prioritised over toughness. In these applications, extraction resistance and dimensional reproducibility after sterilization are central. Steam autoclave exposure at 121 °C for 20 min is possible for unfilled homopolymer PP but can produce post-cycle shrinkage of 0.3–0.8% due to secondary crystallization; this must be included in tooling allowance. Gamma irradiation at 25 kGy can induce radical generation and embrittlement in unstabilized polypropylene, so irradiation-stabilized resin versions are required if terminal sterilization is specified. The base H 400 grade may not be suitable for high-energy electron-beam or gamma sterilization unless the supplier confirms the stabilization package has been formulated for radiation resistance.

In production-scale thin-wall container moulding, H 400 is usually evaluated in 48- to 96-cavity hot-runner tooling with sequential valve-gate opening times of 0.1–0.3 s per gate. Melt is injected at 220–250 °C, and the part is ejected when the skin layer reaches a temperature below the crystallization onset. Short-shot studies are used to map the lower-pressure boundary of the cavity filling, while gate-seal studies define the hold-pressure time needed to prevent backflow and sink. On prototype single-cavity tools, the most frequent processing bottleneck is not melt flow but mould surface temperature variation; a variation of more than ±5 °C across the core and cavity surfaces produces non-uniform skin solidification and differential shrinkage in H 400. For this reason, production tools with water-line circuits spaced at 30–50 mm pitch and turbulent flow velocities above 2.5 m/s are preferred.

For caps and closures, continuous compression moulding is not recommended as a direct drop-in for H 400 because the lack of shear orientation in the hinge region can lower hinge lifetime. Injection moulding remains the dominant conversion route. When a homopolymer cap weight must be reduced below 1.5 g, wall thickness drops under 0.8 mm, and the processing window narrows; under these conditions, process capability studies on 64-cavity tools show higher reject rates from sink marks and hinge cracking if hold-pressure decay is not tuned within ±10% of the gate-seal pressure. Published data for this specific H 400 configuration is limited, so tooling trials remain necessary for critical closure dimensions.