On the procurement shortlist for high-temperature insulation, flexible electronics, and premium packaging, polyimide (PI) film and polyester (PET) film are constantly compared side by side. Both are high-performance polymer films, yet they differ sharply in service temperature, cost, and reliability. This article puts procurement decisions first and uses comparison data to help you lock in the right material fast. (Values reference typical grades such as Kapton HN / Mylar and are representative; confirm against the specific grade datasheet.)
1. Material Overview
PI film (the Kapton family) is made by polycondensation of dianhydrides and diamines. Its rigid, aromatic-rich molecular chains deliver best-in-class heat resistance and dimensional stability, making it the benchmark for flexible printed circuits (FPC) and aerospace insulation. PET film (the Mylar family) is polymerized from terephthalic acid and ethylene glycol. With extremely high cost-performance and huge output, it is the workhorse for labels, packaging, and ordinary electrical insulation.
2. Key Performance Parameter Comparison
| Parameter | PI Film | PET Film |
|---|---|---|
| Density (g/cm³) | 1.42 | 1.39 |
| Continuous service temp (°C) | −269 ~ 260 | ≤ 130 (Classe B) |
| Short-term temp (°C) | 300 ~ 400 | 150 ~ 180 |
| Tensile strength (MPa) | 170 ~ 230 | 150 ~ 220 |
| Elongation at break (%) | 70 ~ 100 | 60 ~ 120 |
| Tensile modulus (GPa) | 2.4 ~ 3.0 | 3.5 ~ 5.0 |
| CTE (ppm/°C) | 12 ~ 20 | 15 ~ 25 |
| Dielectric strength (kV/mm) | 200 ~ 300 | 150 ~ 300 |
| Volume resistivity (Ω·cm) | >10¹⁷ | >10¹⁶ |
| Dielectric constant (1 kHz) | 3.4 | 3.0 ~ 3.3 |
| Water absorption (%) | ~1.3 | 0.4 ~ 0.6 |
| Limiting oxygen index LOI (%) | 37 ~ 47 | 20 ~ 22 |
| Flame rating | UL94 V-0 (inerente) | Requer modif. p/ V-0 |
| Typical thickness (µm) | 12.5 ~ 125 | 1.5 ~ 350 |
| Reference price (USD/kg) | 50 ~ 150 | 2 ~ 8 |
3. In-Depth Performance Comparison
Temperature (the core gap): PI serves continuously from −269 to 260 °C and short-term up to 400 °C; PET tops out around 130 °C (Class B) and survives only 150–180 °C short-term. Above 130 °C PET softens and fails rapidly.
Mechanics: PI retains most of its strength even at 260 °C; PET’s room-temperature strength is comparable to PI and its modulus can be higher, but it drops sharply above 130 °C.
Electrical: Both exceed 150 kV/mm dielectric strength and 10¹⁶ Ω·cm volume resistivity; some PET grades show slightly higher dielectric strength. PI holds better dielectric stability under high temperature and humidity.
Dimensional & flame: PI CTE is about 12–20 ppm/°C and is inherently flame-retardant (LOI 37–47%, UL94 V-0). PET CTE is similar but LOI is only 20–22%, requiring FR modification to reach V-0.
Chemical: Both resist most solvents. PI is slightly weaker against strong alkalis; PET is attacked by strong bases and ester/ketone solvents.
4. Application Scenario Analysis
PI film: FPC base substrate, motor phase insulation, wire and cable wrap, aerospace multilayer thermal insulation (MLI), semiconductor carrier tape, high-temperature masking tape.
PET film: food and industrial packaging, labels and release film, general electrical insulation (≤130 °C), capacitor dielectric, display optical film, lamination and protective film.
Overlap: For mid- to low-temperature uses such as motor slot liners and ordinary cable marking, the two are interchangeable; PET wins on cost.
5. Cost-Benefit Assessment
PET raw material runs about USD 2–8/kg; PI film about USD 50–150/kg — a 10- to 20-fold gap. But if service temperature exceeds 130 °C, PET ages and fails early, and replacement plus downtime cost usually far exceeds the material saving. Rule of thumb: service temperature ≤130 °C with ordinary life expectancy → PET is highly cost-effective; above 150 °C or requiring 10+ years maintenance-free → PI’s total lifecycle cost is actually lower.
6. Selection Recommendations
- Long-term service above 150 °C, or short-term exposure above 300 °C → choose PI.
- Flexible circuits, bend cycles above 10⁶, high-reliability interconnects → choose PI (IPC-4204 base material).
- Cost-sensitive, temperature ≤130 °C, appearance/printing priority → choose PET.
- Need UL94 V-0 without FR modification → choose PI.
- Capacitors, labels, packaging, optical film → choose PET.
- Aerospace, military, semiconductor cleanroom → prefer PI.
7. Conclusion and Action Plan
PI and PET are not simply a high-end versus low-end substitution; they divide the work between “extreme conditions” and “extreme cost-performance.” When procuring, first lock the maximum service temperature and design life, then match the budget: use PET in bulk for mid-low temperature to save cost, use PI for high temperature, long life, and high reliability to control risk. Ask suppliers for third-party test reports (ASTM D5213 / ASTM D882, UL 746 yellow card) and run aging validation under your real conditions before placing volume orders.
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