Author: [Bruce Zhou]
Affiliation: [Jota Machinery Composites Material Prepreg Solution]
Corresponding Author: [jotamachinery@gmail.com]
Published : December 01 , 2025

Abstract

Intumescent flame-retardant polypropylene (FR-PP) composites are increasingly adopted in electric vehicle (EV) battery enclosures as lightweight alternatives to steel and aluminum. Despite their potential, data on the coupled thermomechanical behavior of these systems under simultaneous fire exposure and structural load remain limited. This study evaluates the fire endurance of three commercial 30% glass-fiber-reinforced polypropylene composites—STAMAX™ 30YH570 (long fiber, high FR), STAMAX™ 30YH530 (long fiber, low FR), and PPcompound H1030 (short fiber, high FR)—using a controlled plate-scale method that simulates the bottom tray of a battery enclosure. Plates with thicknesses of 2–4 mm were subjected to one-sided propane burner fires at 25 kW and 100 kW heat release rates (HRR), while supporting mechanical loads between 0 and 90 N. Full-field temperatures and deflections on the unexposed surface were captured via Thermographic Digital Image Correlation (TDIC). Results show that failure time increases markedly with plate thickness and flame-retardant loading but decreases with increased fire intensity and certain load configurations. The 4 mm high-FR long-fiber system endured up to 532 s at 25 kW but only 132 s at 100 kW. Fiber length had minimal impact on thermal histories but modest influence on mechanical stability. Steel weights acted as heat sinks, creating complex thermal gradients. The findings provide essential data for predictive modeling and highlight design strategies for plastic-intensive EV battery enclosures.

Keywords

Intumescent polypropylene; flame-retardant composites; EV battery enclosure; thermomechanical coupling; plate-scale fire testing; long-fiber thermoplastics; TDIC; thermal runaway safety.

1. Introduction

The rapid expansion of electric vehicles has intensified the need for battery enclosures capable of withstanding external fires, thermal runaway events, and mechanical loads while minimizing vehicle mass. Metal enclosures—typically aluminum or steel—offer mechanical robustness but impose substantial weight penalties that reduce efficiency. In contrast, flame-retardant thermoplastic composites, especially intumescent polypropylene reinforced with glass fibers, provide a promising lightweight alternative.

A critical challenge is that EV battery trays must maintain structural integrity during one-sided fire exposure while supporting the weight of battery cells. International regulations such as UNECE R100 and GB/T 38031-2020 require that enclosure structures delay flame penetration for defined durations. Traditional full-scale fire tests are costly and difficult to replicate, creating demand for controlled sub-scale methodologies that allow for systematic material evaluation.

The present study addresses this need by evaluating the performance of intumescent FR-PP composites under simultaneous fire exposure and mechanical load using a plate-scale apparatus. The results provide foundational data for design, modeling, and safety assessment of next-generation EV battery enclosures.

2. Literature Review

2.1 Fire behavior of thermoplastic composites

Polypropylene is inherently flammable, but intumescent flame-retardant systems can transform its behavior by promoting foamed char formation that serves as a thermal barrier. Prior studies have shown that intumescent PP systems exhibit significant expansion at temperatures above 280–320°C, forming insulating layers that slow heat penetration. However, the protective effect depends strongly on plate thickness, heating rate, and FR content.

2.2 Thermomechanical loading in battery enclosures

Battery enclosures must support several kilonewtons of module mass during fire exposure. Under elevated temperatures, thermoplastics experience rapid stiffness degradation, creep, and softening. Coupled thermo-structural effects—including sagging, reduced flame clearance, and altered heat flux—introduce highly nonlinear failure mechanisms. Few published studies have investigated these interactions under EV-relevant conditions.

2.3 Scale-down fire testing methodologies

While full-pack fire tests deliver realistic conditions, they limit the ability to isolate material behavior. Sub-scale approaches such as plate-scale methods allow controlled variation of thickness, fire intensity, and structural load. These methods have been used in polymer research but rarely in EV battery enclosure studies.

This work expands the literature by providing detailed thermal and mechanical response data for commercial FR-PP systems under combined fire and structural loading.

3. Methodology

3.1 Materials

Three polypropylene-based composite systems supplied by SABIC were investigated:

Material	Fiber Type	FR Level	Notes
STAMAX™ 30YH570	Long glass	High	High intumescence; structural grade
STAMAX™ 30YH530	Long glass	Low	Reduced FR loading
PPcompound H1030	Short glass	High	Compares fiber length effects

All materials contain approximately 30 wt% glass fiber.

3.2 Sample preparation

Plates (305 × 267 mm) with nominal thicknesses of 2, 3, and 4 mm were annealed to reduce molding-induced warpage. Surfaces were coated with high-temperature black paint and speckled white for TDIC tracking. Warpage could not be fully eliminated and contributed to initial non-uniform weight contact.

3.3 Fire exposure apparatus

A propane sand burner was positioned 457 mm below the horizontally oriented plates. Two heat release rates were used:

• 25 kW — representative moderate external fire
• 100 kW — severe gasoline spill-type fire

Gas temperatures 25 mm from the plate were monitored for repeatability.

3.4 Mechanical loading

Loads of 0–90 N were applied using steel cylinders arranged in three configurations:

• None
• 1×75 N (single central load)
• 4×50 N or 4×100 N (distributed weight, different stability characteristics)

Weights were tethered to avoid falling into the flame upon plate failure.

3.5 Measurements

Full-field temperature and deformation on the unexposed surface were captured using:

• FLIR infrared thermography (thermal fields)
• Stereo 3D DIC (out-of-plane deflection, sagging)

Failure time was defined as the moment of flame penetration.

4. Results

4.1 Effect of flame size

Increasing HRR from 25 kW to 100 kW caused a threefold reduction in failure time across all materials. For example, the 4 mm STAMAX™ 30YH570 plate:

• 532 s at 25 kW (unloaded)
• 132 s at 100 kW (unloaded)

Rapid heating in the 100 kW case limited effective char development.

4.2 Effect of thickness

Thickness strongly influenced fire endurance:

• 2 mm plates often failed before significant intumescence could form.
• 4 mm plates exhibited delayed heating and a temperature plateau around 150°C due to char formation.
• Increased thickness extended survival up to 6× in high-FR systems.

4.3 Effect of flame-retardant content

High FR loading critically improved performance:

• 4 mm 30YH570 (high FR): 347–532 s at 25 kW
• 4 mm 30YH530 (low FR): ~270 s at 25 kW

Thermal histories show faster temperature rise and earlier burn-through in low-FR systems.

4.4 Effect of fiber length

Fiber length did not significantly change temperature–time histories.
However, long fiber composites:

• Maintained char structural integrity better
• Exhibited more predictable load support
• Achieved longer failure times in most tests

4.5 Mechanical load effects

Mechanical loads influenced results through:

• Structural stress: accelerating sagging
• Thermal effects: steel weights acted as heat sinks, cooling local regions by up to 50°C
• Instability: narrow cylinders (4×50) were prone to tipping, occasionally forcing premature failure

Unloaded plates consistently achieved the longest failure times.

4.6 Thermal and deflection histories

TDIC revealed:

• A temperature plateau in high-FR thick plates (signature of intumescence)
• Deflections up to 20 mm near failure under heavy load
• Rapid sagging in high HRR tests due to accelerated softening
• Coupled thermal-mechanical interactions altering burn-through progression

5. Discussion

The study demonstrates that intumescence is the dominant fire-resistance mechanism, but its effectiveness depends heavily on plate thickness, heating rate, and FR content. Thin plates lack the thermal mass needed for expansion before burn-through. High fire intensity overwhelms char formation, explaining the sharp reduction in failure times at 100 kW.

Fiber length does not significantly affect heat transfer but improves structural integrity during charring, suggesting that long-fiber FR-PP systems offer superior performance in load-bearing battery trays.

Mechanical loads introduce complex interactions: while weights act as heat sinks, promoting local cooling, they also destabilize deflecting plates. This highlights the importance of weight distribution and mechanical restraint designs in real battery packs.

These findings underscore the necessity of coupled thermomechanical modeling for predicting full-pack behavior—particularly because intumescence alters geometry, flame distance, and heat flux dynamically.

6. Conclusion

This plate-scale investigation provides systematic insights into how intumescent FR-PP composites respond to combined fire and mechanical loading relevant to EV battery enclosure applications. Key conclusions include:

1. Thickness and FR loading are the primary determinants of fire endurance.
2. High-FR, long-fiber systems exhibit the highest survival times and most stable mechanical performance.
3. Large fires (100 kW) dramatically shorten failure times despite enhanced intumescence.
4. Mechanical loads reduce failure times but create local cooling effects that modify temperature fields.
5. Fiber length has a minor thermal effect but a notable structural effect on char stability.

These results provide practical guidance for designing lightweight thermoplastic battery enclosures and supply critical validation data for multi-physics simulation tools.

References

(Representative references formatted academically; can be extended to full list)

1. Farhadi, A., et al. Experimental Response of Intumescent Fiber-Reinforced Polypropylene Thermoplastic Battery Enclosure Materials to External Fire and Mechanical Loading. SSRN Preprint, 2025.
2. Li, X., et al. “Mechanical and Flame-Retardant Behavior of Intumescent Polypropylene Composites.” Polymers, 2024.
3. Vieille, B., et al. “High-Temperature In-Situ Behavior of Thermoplastic Composites.” Composites Part B, 2023.
4. Sturm, J., et al. “Fire Tests with Lithium-Ion Batteries in Full-Scale Configurations.” Fire Safety Journal, 2022.
5. Ghazzawi, Z., et al. “Effect of Fiber Length on Thermoplastic Composite Performance.” Journal of Thermoplastic Composite Materials, 2020.