Is Polyester Fabric Flammable? A Detailed Analysis from ‘Flammable’ to ‘Flame Retardant

Polyester is one of the most widely used synthetic fibers in the world, known for its durability, versatility, and affordability. It is the foundational material for a vast range of products, found in everything from clothing (like shirts and activewear) and home furnishings (such as curtains and bedding) to industrial applications like ropes and tarps. However, when it comes to fire safety, not all polyester is created equal.

Because of its wide use, it’s critical to understand its fire performance. We must first clarify a core premise: are we talking about “Regular Polyester (Non-FR Polyester)” or “flame-retardant polyester (FR-PET)”? These two materials are completely different in their fire safety ratings; the former is flammable, the latter is flame-retardant.

Regular polyester (PET) itself is flammable and is a “melt-combustible” material. In contrast, flame-retardant polyester achieves its self-extinguishing property through technical modification. This article will use scientific metrics to detail the essential differences between these two types of polyester.

Key Takeaways:

• Regular Polyester (PET) is Flammable: It has a Limiting Oxygen Index (LOI) of just 20-21%, which is enough to burn in normal air (21% oxygen) and will melt and drip.
• Flame Retardant (FR-PET) is Self-Extinguishing: Technical modification raises its LOI to >28%, meaning it cannot sustain combustion in normal air and will self-extinguish.
• LOI is the Key Metric: The LOI “score” determines flammability. Anything requiring >26% oxygen is generally considered flame retardant.

Regular Polyester (Non-FR Polyester) is a Flammable Material

Chemical Structure and Ignition Point

Non-FR polyester (PET) is a thermoplastic synthetic fiber formed by the polycondensation of terephthalic acid (TPA) and ethylene glycol (EG). It does not have natural flame retardancy and is classified as a combustible fiber. Its ignition point is approximately 485–560°C, and its auto-ignition point is around 630°C.

Burning Characteristics: Melting and Dripping

The burning characteristic of non-FR polyester is “melt-combustion.” When exposed to fire, it will first soften, melt, shrink, and drip. These hot, molten drips can cause secondary ignition if they fall onto other flammable materials, or cause severe burns. After the flame source is removed, it will typically continue to burn for a short time before extinguishing; it is not self-extinguishing.

The Scientific Basis: Limiting Oxygen Index (LOI)

The Limiting Oxygen Index (LOI) is one of the most common and intuitive scientific metrics for judging a material’s fire performance. Think of it as a “fire safety score”—the higher the score, the safer the material.

First, Understand LOI’s Role: What Does It Tell Us?

The role of LOI is very simple: it tells us how “dependent” a material is on oxygen to burn.

We all know that fire requires oxygen. The air we breathe contains about 21% oxygen. This 21% is the “passing grade.”

What Does This “Score” (The Number) Mean?

The LOI number (or “score”) represents: “The minimum percentage of oxygen a material needs to ignite and keep burning.”

Comparing this score to 21% (the oxygen in the air) makes its meaning crystal clear:

• LOI > 26% (High Score = Flame Retardant): This value means it needs more than 26% oxygen to burn.
  In simple terms: The air only has 21% oxygen, which is “not enough” for it to burn. So, in normal air, it’s either very hard to ignite, or if the flame source is removed, it extinguishes on its own. This is called “flame retardant.”
• LOI ≤ 21% (Low Score = Flammable): This value means it only needs 21% or even less oxygen to sustain combustion.
  In simple terms: The oxygen in the air is “more than enough” for it to burn. So, once ignited, it can use the air’s oxygen to keep burning. This is called “flammable.”

Polyester’s LOI Position: Just “Flammable”

Based on this standard, non-FR polyester (PET) has an LOI of about 20% to 21%.

This number clearly classifies it as a flammable fiber. Because the amount of oxygen it needs to sustain combustion is equal to (or even lower than) the oxygen content in the air, it can burn continuously in air.

Burning Characteristics of Non-FR Polyester

When non-FR polyester burns, it exhibits the following typical characteristics, which are completely different from natural fibers like cotton, regarding whether cotton fabric is flammable, you can read: Is Cotton Fireproof? Unveiling the Truth About Cotton’s Flame Retardancy.

Property	Behavior
Burning State	Melts and drips, accompanied by black smoke
Flame Color	Yellow with a blue edge
Odor	Aromatic scent (similar to burnt plastic)
Residue	Black, hard, bead-like lumps (carbonized drips)
Self-Extinguishing Property	Not self-extinguishing (continues to burn briefly after flame is removed)

Comparison: LOI and Flammability of Polyester vs. Other Fibers

Comparing non-FR polyester with flame-retardant polyester and other common fibers clearly shows its flammable classification:

Material	LOI %	Flammability Rating	Burning Behavior
Cotton	17–18	Flammable	Burns quickly, no melting/dripping, grayish-white smoke
Non-FR Polyester (PET)	20–21	Flammable	Melts and drips, black smoke
Acrylic (non-modified)	18-20	Very Flammable	Bright flame, burns rapidly
FR Viscose	26–30	Flame Retardant	Self-extinguishes when flame is removed
Aramid	≥ 28–30	Flame Retardant	Does not melt or drip, self-extinguishes
FR Polyester (FR-PET)	≥ 28–35+	Flame Retardant / Self-Extinguishing	No molten drips, extinguishes when flame is removed

Flame Retardant Modification Technology for Polyester

To transform flammable non-FR polyester (LOI 20-21%) into flame-retardant polyester (LOI > 28%), specific technical treatments are required. There are two main methods:

1. Flame Retardant Finishing (Topical Treatment)

This is a technique where flame retardants (such as phosphorus- or halogen-based) are “added” to the surface of the finished fabric through dipping, coating, or resin bonding.

• Advantages: Lower cost, mature process.
• Disadvantages: Not durable to washing. The retardant agents are lost over repeated launderings, causing the flame-retardant effect to diminish or fail.

2. Inherent Flame Retardant (Inherent FR) Modification

Also known as solution-dyeing or co-polymerization modification. This involves introducing flame-retardant monomers (like phosphorus- or nitrogen-based compounds, e.g., DOPO, CP) into the polyester’s molecular chain during the polymerization (pre-spinning) stage.

• Advantages: The flame-retardant property becomes a permanent part of the fiber. It is therefore extremely durable to washing, and the effect will never diminish. It typically achieves self-extinguishing and no-drip performance.
• Disadvantages: Higher cost than topical finishing.
• Typical Brands: Begoodtex® IFR Polyester.

Conclusion

Based on the analysis above, the conclusion regarding polyester’s fire performance is clear:

1. Non-FR Polyester (PET): Is a flammable material. Its LOI is only 20-21%, just enough to burn in air. Its burning characteristic is melting and dripping with black smoke, which poses a fire hazard.
2. Flame Retardant Polyester (FR-PET): Is a flame-retardant material. Through inherent modification or finishing, its LOI is raised to 28% or higher, allowing it to self-extinguish and not drip, meeting strict fire standards like B1 or NFPA 701.

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