If you’ve ever stood next to a jet engine on a tarmac, leaned against a high-temperature furnace control panel, or held a flexible circuit board in a hybrid electric car, chances are you’ve come into contact with polyimide (PI) without even realizing it. As someone who’s spent the last 12 years working with polyimide resins and films—first as a materials engineer testing extreme environments, and now as part of a small, specialized PI supply team—I get asked this question at least once a week: “What makes polyimide such a go-to for flame-resistant applications?” It’s not just marketing hype, either. The flame retardancy of polyimide is a result of decades of material testing, real-world field data, and a molecular structure that’s fundamentally built to withstand heat. Today, I want to break down what that actually means, why it matters for industries that can’t afford fire risks, and how we work with clients to tailor PI solutions for their unique needs. Polyimide
Let’s start with the basics of molecular structure, because that’s where polyimide’s flame resistance starts. Unlike common polymers like polyethylene or even nylon, which have long chains of repeating carbon and hydrogen units connected by weak single bonds, polyimide’s backbone is strung with aromatic rings and imide groups—those five-membered rings made of two carbon atoms, two nitrogen atoms, and one oxygen. This structure is already way more stable than most other plastics. When you heat PI, those strong aromatic bonds don’t break down easily at low temperatures. In fact, the initial decomposition temperature of standard polyimide is around 550°C (1022°F), which is almost twice the melting point of many other engineering plastics that start degrading at 200–300°C. Compare that to, say, polyester, which will catch fire and burn continuously once exposed to a flame, releasing toxic fumes as it breaks down. Polyimide? It doesn’t do that easily.
I still remember the first time my old lab team tested PI film for a aerospace client back in 2012. We took two 1×1 inch samples: one of our standard 25-micron PI film, and one of a leading competitor’s flame-retardant polyester film. We held both over a Bunsen burner flame for 10 seconds, then pulled them away. The polyester sample immediately started burning, dripping molten plastic (which is a huge fire risk, by the way—those drips can spread flames to nearby materials) and producing thick black smoke. The PI sample? It charred on the surface, but stopped burning within a second of being removed from the flame. No drips, no ongoing combustion, just a stable carbonized layer. That carbon layer isn’t a side effect—it’s the core of PI’s flame retardancy. When PI is exposed to high heat or flame, the surface decomposes to form a dense, continuous char that acts like a barrier. It keeps oxygen out, prevents heat from reaching the unburned polymer below, and traps any volatile gases that would otherwise feed the fire. Most other flame-resistant polymers rely on additives—things like bromine or phosphorus compounds mixed into the material—to stop burning. Those additives can leach out over time, break down at high temperatures, or release toxic fumes when they do work. PI’s flame resistance is inherent, built into the molecules, so it doesn’t need extra chemicals to perform. That’s a big selling point for industries that prioritize long-term safety and low toxicity: medical device makers, aerospace engineers, and electric vehicle teams all avoid additive-based flame retardants because they can degrade or create harmful byproducts.
Of course, “flame retardant” isn’t a one-size-fits-all term, and not all polyimides are the same. Over the years, we’ve developed specialized PI grades for different applications, each with slightly different flame performance. Let’s walk through the main types, because that’s where most of the confusion comes in. First, there’s the standard, unmodified aromatic polyimide—the kind we use for flexible circuit boards and thermal insulation wires. Its limiting oxygen index (LOI) is around 38%. Wait, what’s LOI? That’s the percentage of oxygen in the air that’s required to keep a material burning. Most common plastics have an LOI between 15 and 21%, which means they’ll burn easily in regular air (air is 21% oxygen). A material with an LOI above 28% is considered highly flame retardant. So standard PI at 38% means it won’t burn in normal air, and only supports combustion in atmospheres with unusually high oxygen levels—like the inside of a spacecraft, or a deep-sea sub’s electrical system. That’s why NASA and SpaceX have used polyimide insulation for wiring on rockets for decades.
Then there’s high-temperature polyimide, like the ones we make for furnace linings or jet engine thermal shields. These have even higher LOIs, up to 45%, and their char layer is even more stable at temperatures above 1000°C. I once worked with a steel manufacturing client who used our high-temperature PI fabric as a protective layer for their furnace control cables. They tested it by exposing the fabric to 1200°C molten slag for 15 minutes—most other polymer fabrics would have burned through in 30 seconds, but the PI fabric’s char layer stayed intact, protecting the cables inside. That’s the kind of performance you can’t get with additives.
We also work with clients who need PI that’s both flame retardant and flexible—for wearable electronics, for example, or foldable screens. Flexible polyimide films have a slightly lower LOI, around 32%, but they’re still far more flame resistant than alternatives like polycarbonate or even silicone. Last year, a client developing fire-resistant smart gloves for first responders came to us because their initial silicone-based material melted when exposed to a small heat spark. We supplied them with our flexible PI film, and when they tested it, the sample charred instead of melting, and stopped burning immediately after being removed from the flame. It still bent 10,000 times without cracking, which is exactly what they needed.
But here’s the thing about PI’s flame retardancy that’s important to mention: it’s not “fireproof.” If you expose PI to enough heat for long enough, it will eventually decompose, and under the right conditions, it can burn. But the key difference is how it behaves compared to other materials. PI doesn’t produce molten drips that spread fire, it releases much less smoke and toxic fumes than additive-based polymers, and it has a much higher threshold before it starts to degrade. For industries where fire safety isn’t a box-ticking exercise—it’s a matter of lives, equipment, and avoiding catastrophic failure—that difference is everything.
I’ve seen a lot of misconceptions over the years, too. Some engineers hear “polyimide” and think it’s too expensive, or that it’s hard to work with. But the flame retardancy of PI actually saves money in the long run. For example, in electric vehicles, battery packs require fire-resistant materials to contain thermal runaway, a rare but dangerous event where a battery cell overheats and catches fire. Using additive-based flame retardants can add weight, degrade over the lifetime of the vehicle, and require extra layers of insulation. PI’s inherent flame resistance means you can use thinner, lighter materials that last longer, reducing overall vehicle weight (which improves range) and cutting maintenance costs. We worked with an EV manufacturer in Germany a couple of years ago to replace their additive-based insulation with our PI film; they reported a 12% reduction in the weight of their battery pack’s fire protection system, and no flame-related issues in 18 months of field testing.
Another common question: how does PI perform in terms of smoke and toxicity when it does burn? This is a big one for indoor applications, like aircraft cabins, building insulation, or medical devices. When most polymers burn, they release a mix of carbon monoxide, cyanide, and other toxic fumes that are the leading cause of fire-related deaths, not the flames themselves. Studies have shown that when polyimide burns, the smoke density is 50–70% lower than that of halogenated flame-retardant polymers, and it releases 30–40% less carbon monoxide. That’s a huge deal for public spaces where people need time to evacuate in a fire. We test all our PI grades per ASTM E662, the standard for smoke density in building materials, and our flame-retardant PI meets the strict Class A requirements, the highest rating for low smoke emission.
Over the last decade, we’ve also been adapting PI’s flame retardancy for new applications, like renewable energy systems. Solar panel inverters and wind turbine generators operate in harsh, dusty, high-temperature environments, and they need insulation that won’t catch fire if there’s an electrical fault. Our latest grade of PI insulation for inverters has an LOI of 40%, and has passed 10,000 hours of continuous testing at 180°C with no degradation in flame performance. We’re also working on a new flexible PI foam for electric vehicle battery enclosures, which combines PI’s inherent flame resistance with the light weight of foam—something that would have been impossible to achieve with additive-based flame retardants a few years ago.
I want to be honest, though: polyimide isn’t the right choice for every application. If you’re looking for a low-cost, one-time-use material that only needs to resist small amounts of heat, PI is overkill, and it would be a waste of money. But if you’re working in aerospace, automotive, medical, industrial manufacturing, or any field where fire risk is a critical safety concern, PI’s flame retardancy is one of the most reliable materials you can use. It’s not a “fix” for bad design, but it’s a layer of protection that doesn’t wear out, doesn’t add toxic chemicals to your product, and performs consistently across extreme conditions.
As a PI supplier, my team and I work closely with every client to match the right PI grade to their specific needs. We don’t just send generic PI film; we test samples for their LOI, smoke emission, tensile strength, and thermal stability, and adjust the formulation if needed. If you’re working on a project where flame resistance is a top priority—whether it’s a new medical device, a jet engine component, an electric vehicle battery, or a building fire barrier—we can help you find a PI solution that meets your requirements.
If you’re interested in learning more about our polyimide grades, testing protocols, or how PI can improve the flame safety of your product, feel free to reach out to our team for a consultation. We can send free samples for testing, walk you through our performance data, and help you integrate polyimide into your design process.
Boric Acid Intermediates References
- Lian, G., et al. (2018). Flame retardant mechanisms of aromatic polyimides. Polymer Degradation and Stability, 155, 112-122.
- National Aeronautics and Space Administration (NASA). (2020). Polyimide materials for aerospace applications. NASA Technical Report TP-2020-220017.
- American Society for Testing and Materials (ASTM). (2021). Standard test method for specific optical density of smoke from burning materials (ASTM E662-21).
- Zhang, Y., et al. (2022). Inherent flame-retardant polyimides for electric vehicle battery insulation. Journal of Applied Polymer Science, 139(18), 52147.
- Underwriters Laboratories (UL). (2019). Standard for flame protection in electrical equipment (UL 94).
Hubei Jiutian Bio-medical Technology Co., Ltd.
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