Polypropylene (PP) in EV battery components—cell cases and modules—is specified where mass reduction, chemical resistance to carbonate-based electrolytes, and high-volume injection moulding economics intersect with flame-retardant and dielectric requirements. Neat PP homopolymer has a density of 0.90–0.92 g/cm³ (ISO 1183-1) and a tensile modulus near 1,500 MPa (ISO 527-2), but its heat deflection temperature under 1.8 MPa load (ISO 75-2) is limited to approximately 50–60 °C. Cell cases positioned adjacent to lithium-ion cells that operate at 40–60 °C therefore require mineral-filled, glass-reinforced, or impact-modified PP compounds rather than commodity homopolymer grades. In prismatic cell carriers, wall thicknesses are typically 1.5–3.0 mm, and the moulded parts must pass UL 94 vertical burn testing at the same thickness where flame retardance is specified. Compliance under EU RoHS Directive 2011/65/EU and REACH Regulation 1907/2006 is also required for cell case and module enclosure materials.

Why Do Module Designers Select Heterophasic Copolymer PP Over Homopolymer for Cell Retainers?

Heterophasic copolymer PP contains a dispersed ethylene-propylene rubber phase that raises notched Charpy impact values to 20 kJ/m² or more at 23 °C when tested to ISO 179-1/1eA, compared with 3–6 kJ/m² for a homopolymer PP of similar melt flow rate. This difference is critical at snap-fit retention features, terminal slots, and cell compression ribs where sharp radii and moulded-in stresses initiate brittle fracture in low-temperature impact. At -30 °C, the ductile-to-brittle transition shifts to lower impact values, but heterophasic grades with an ethylene-octene modifier content of 15–20 wt% maintain more than 8 kJ/m² notched impact. The melt flow rate measured to ISO 1133-1 at 230 °C/2.16 kg is usually specified between 8 g/10 min and 25 g/10 min for thin-wall cell case filling; higher melt flow improves flow length but reduces impact toughness and can promote flashing at multi-cavity tools with clamp forces below 1,000 t.

Flame-retardant PP compounds for module enclosures are frequently formulated with halogen-free intumescent systems based on ammonium polyphosphate and a carbonific char former. Twin-screw compounding on a co-rotating extruder with an L/D ratio of 40:1 is used to disperse the additives while limiting melt temperature. If the melt temperature exceeds 240 °C during compounding or injection moulding, the intumescent package can undergo premature esterification and gas evolution; moulded parts then display surface splay, voids, and non-uniform UL 94 V-0 performance. Production-scale observations indicate that screw speed and back pressure must be reduced relative to unfilled PP to keep melt temperature below 220 °C, and residence time should be held below 4 min to prevent additive decomposition. Because the flame-retardant rating is thickness-dependent, parts moulded with wall thickness below 1.5 mm may require a higher additive loading or a brominated-phosphorus synergistic package to achieve V-0; however, brominated systems can lower comparative tracking index and may be restricted under EU RoHS/REACH conditions.

Electrolyte Exposure and Thermal Ageing Boundaries in Cell Case Assemblies

Polypropylene displays low weight gain in carbonate-based lithium-ion electrolyte solvents such as ethylene carbonate, dimethyl carbonate, and diethyl carbonate when tested according to ASTM D543 at 23–40 °C. This chemical resistance supports PP use in cell spacers, end plates, and busbar carriers that may see electrolyte vapour. Continuous exposure above 80 °C changes the behaviour: swelling increases, retained tensile strength falls, and creep under constant load becomes measurable. Heat-stabilised 20 wt% talc-filled PP grades evaluated under ISO 188 at 100 °C for 1,000 h typically retain more than 85–90 % of original tensile strength, but published data for this specific configuration is limited, particularly when flame-retardant additives are present. The upper continuous-use boundary for PP in a cell case is therefore set by the combination of heat deflection under load, long-term thermal oxidation, and absorption of electrolyte decomposition products; glass-reinforced PP with heat stabilisers can move the practical limit closer to 110 °C in low-stress areas, but direct contact with cells operating above 60 °C requires derating of structural loads.

Busbar carriers and module side plates in high-voltage packs must maintain creepage and clearance distances under IEC 60664-1; the comparative tracking index measured to IEC 60112 is therefore a primary electrical specification. Halogen-free FR PP compounds with mineral filler often report CTI 600 V at 3.0 mm, while some glass-reinforced and brominated FR packages fall to 400–500 V. Volume resistivity measured to ASTM D257 is generally 10^14–10^16 Ω·cm at 23 °C, and dielectric strength to IEC 60243-1 is typically 20–30 kV/mm for a 2.0 mm dry plaque. After humid ageing at 85 °C/85 % RH for 1,000 h, surface resistivity can drop by several orders of magnitude, so unsealed PP parts in condensation-prone module areas are evaluated at end-use humidity rather than only at ambient laboratory conditions. These electrical tests are performed on conditioned specimens because moisture uptake in the additive package, not the PP matrix, dominates the decline in insulation resistance.

When Long-Glass-Fibre PP Replaces Die-Cast Aluminium in Module Housings

Long-glass-fibre PP with glass content of 30–40 wt% achieves tensile modulus values of 5,000–6,500 MPa under ISO 527-2 and notched Charpy impact above 20 kJ/m² at 23 °C. These values permit replacement of die-cast aluminium in module end plates and lower-load structural frames, reducing part mass by approximately 30–40 % at equivalent wall section. The dominant manufacturing variable is fibre attrition. Pultruded long-glass pellets with initial fibre length near 12 mm can be reduced to less than 2 mm in the moulded part if a high-compression general-purpose screw is used; fibre length retention above 4 mm is typically achieved only with low-compression barrier screws and low-shear check-ring assemblies on injection units of 80–120 mm screw diameter. Gate diameter below 3.0 mm and injection velocity above 100 mm/s promote jetting, glass-fibre separation, and weld-line weakness at boss features. These process limitations impose tooling constraints: fan gates, short flow lengths, and gentle transitions in runner diameter are required to maintain mechanical performance in large module housings.

Insert moulding of brass or copper busbars into PP module housings introduces residual hoop stress because PP shrinkage is significantly greater than that of the metal insert. Unfilled PP exhibits mould shrinkage of 1.2–1.8 % measured to ISO 294-4, while 30 wt% long-glass-fibre PP can show anisotropic shrinkage of 0.2–0.6 % in the flow direction and 0.5–1.0 % transverse. After thermal cycling from -40 °C to 85 °C, stress concentration at the insert periphery can produce microcracks if the insert diameter exceeds 2.5–3.0 times the boss diameter. Production-scale failure modes observed in module housings include radial cracks initiating at insert knurls and sink marks at the opposite face due to insufficient packing pressure. To reduce these failures, processors select impact-modified PP grades with high molecular weight ethylene-octene elastomer, maintain mould temperatures between 30 °C and 60 °C, and apply pack pressure above 60 MPa for a duration sufficient to gate freeze.

The following table compares typical property ranges for PP formulations relevant to cell cases and module enclosures; values are based on publicly available supplier datasheet ranges and are not single-lot guarantees.

The Stable Processing Window for Intumescent FR PP Narrows Above 230 °C

The stable processing window for halogen-free intumescent PP compounds is narrower than for neat or talc-filled PP. Melt temperature measured at the nozzle should be maintained between 200 °C and 220 °C for ammonium polyphosphate–pentaerythritol systems; below 200 °C, viscosity rises and filling of 1.5 mm cell-case ribs becomes incomplete, while above 230 °C, surface splay and plate-out increase sharply. Screw speed should be set below 100 rpm on a 70 mm diameter reciprocating screw, and back pressure should not exceed 0.5 MPa to limit shear heating. Mould temperatures of 30–50 °C are common; higher mould temperatures improve knit-line strength but prolong cycle time and may allow additive bloom on the cavity surface. These compounds should be pre-dried at 80 °C for 2–3 h in a desiccant dryer after open storage at relative humidity above 60 %, because the hygroscopic intumescent additive package can generate steam marks and reduce flame-retardant efficacy. The use of amine-based antioxidant packages in combination with some intumescent FR systems should be avoided because acid-base interactions can reduce char formation during combustion and increase plate-out during moulding.

Weld-line strength in mineral-filled PP cell cases is controlled by filler orientation and packing time. Talc platelets align parallel to the flow front and reduce weld-line tensile strength to 60–70 % of the bulk value when measured to ISO 527-2 on double-gated tensile specimens. Long-glass-fibre PP shows lower weld-line tensile retention, often below 50 %, because fibre orientation is discontinuous across the meeting flow fronts. In cell cases with multiple holes for terminal bushings and busbar screws, the number of weld lines can be reduced by sequential valve gating; where weld lines are unavoidable, local thickness should be increased by 0.3–0.5 mm and mould temperature raised to 50–60 °C to improve polymer diffusion at the knit line. Processing studies on hot-runner moulds with 8 drops for prismatic cell carriers show that fill time and pack pressure profile are stronger levers than melt temperature for avoiding weak knit lines at framed cell pockets.

Polypropylene has low thermal conductivity, typically 0.15–0.22 W/m·K measured to ISO 22007-2; glass and mineral fillers can raise this to 0.30–0.45 W/m·K. This limits use in direct-cooled module housings unless thermally conductive ceramic or graphite fillers are added, which increases compound cost and reduces impact strength. In cell cases, low thermal conductivity is less critical because heat transfer is dominated by cell-to-cell conduction and liquid cooling plates, but module side plates that must dissipate heat from busbars require thermal derating or metal overmoulding.

Dimensional stability of PP cell cases is assessed under ISO 291 conditioning because PP shrinkage and warpage are sensitive to cooling rate, filler orientation, and post-mould crystallisation. For 20 wt% talc-filled PP, mould shrinkage in the flow direction is commonly 0.9–1.1 %, while transverse shrinkage is 1.1–1.4 %; this anisotropy produces bowing in flat cell-case floors unless gating is positioned to create a symmetric flow front. Tooling qualification for series production therefore includes thermal cycling from -40 °C to 95 °C for 500 cycles, followed by dimensional check on a coordinate measuring machine and dielectric withstand testing at 1,500 V AC for 60 s. Cell cases that pass initial mechanical testing can fail after thermal cycling because frozen-in stress relaxes and warpage exceeds the cell-pouch compression tolerance of ±0.2 mm. This operational boundary dictates that PP compounds with high filler content must be moulded with adequate pack pressure and controlled cooling to maintain dimensional stability in production.