Basic knowledge of flame retardant finishing of textiles: principles, methods and applications

Introduction

Most textile fibers are organic polymers, which undergo thermal decomposition when exposed to high temperatures or flames, producing flammable gases and volatile substances that can ignite. With increasingly stringent fire safety requirements, the flame-retardant properties of textiles are receiving growing attention.

In specialized applications such as fire suits, protective clothing, military textiles, aerospace and transportation interiors, stage curtains, hotel décor materials, carpets, children’s products, and industrial textiles, fabrics not only need to meet comfort and durability requirements but also possess a certain degree of flame retardancy. Therefore, imparting safety and protective properties to textiles through flame-retardant finishing has become an important direction in textile finishing technology.

I. What is Flame Retardant Finishing?

“Flame retardant” does not mean that the treated textile will not burn at all, but rather that the material has the ability to delay combustion, reduce the speed of flame spread, and reduce the hazards of combustion.

High-quality flame retardant fabrics typically have the following characteristics:

• They do not ignite rapidly upon contact with a fire source;
• They self-extinguish quickly after being removed from the fire source;
• They reduce the duration of continuous combustion;
• They reduce residual flame (the flame continues to burn after the fire source is removed);
• They reduce the risk of smoldering (the flame does not appear but the fabric continues to char).

Therefore, the core objective of flame retardant finishing is:

To reduce the burning rate of the fiber material, improve the safety of the fabric in a fire environment, and buy time for personnel to escape and for fire control.

II.Combustion Characteristics of Different Textile Fibers

Different fibers exhibit significantly different combustion properties due to variations in chemical structure, molecular composition, and physical form.

1. Natural Fibers

Cotton Fiber

Cotton is a typical cellulose fiber with high flammability.

Combustion Characteristics:

Easily ignited;
Fast burning speed;
Produces a large amount of ash;
May continue burning after being removed from the flame source.

Main Combustion Process of Cotton Fiber:

Cellulose → Thermal decomposition → Combustible gas → Flame combustion

Therefore, cotton fabrics usually require flame-retardant finishing to meet protective application requirements.

Wool Fiber

Wool is a protein fiber with better natural flame-retardant properties compared to cotton.

Characteristics:

Higher ignition temperature;
Slower burning speed;
Easily forms a char layer; Easily self-extinguishes after being removed from the flame source.

2. Synthetic Fibers

Polyester
When polyester burns, it will:

Melt and shrink;
Produce flammable gases;
Molten droplets may cause secondary injury.

Therefore, flame-retardant finishing of polyester mainly focuses on:

reducing the burning rate;

reducing dripping;

increasing the limiting oxygen index (LOI).

3. Nylon

Nylon has a certain degree of flame resistance, but it easily melts and drips during combustion, requiring flame-retardant modification tailored to the specific application.

III.Mechanisms of Flame Retardant Action in Textile 

The combustion of textiles is a complex physical and chemical process. Flame retardants primarily achieve their flame-retardant effect by altering the thermal decomposition, gas release, and oxidation reactions during combustion.

Currently, the main flame-retardant mechanisms include the following:

1.Catalytic Dehydration and Carbonization Mechanism

This mechanism is mainly applied to cellulosic fibers (such as cotton, linen, and viscose).

Under normal conditions, cellulose easily decomposes at high temperatures, producing:

Flammable gases;

Flammable liquids;

Flammable substances such as L-glucose.

Phosphorus-containing flame retardants can promote the premature dehydration and carbonization of cellulose, enabling the fiber to form a stable char layer.

The process is as follows:

Cellulose
↓ Dehydration reaction
↓ Formation of char layer
↓ Reduction of combustible gas release
↓ Reduction of combustion rate

Phosphorus-containing flame retardants are currently the most widely used type in flame-retardant finishing of cotton fabrics.

2.Cas-Phase Flame Retardant Mechanism

Gas-phase flame retardancy is primarily achieved by influencing the chemical reactions in the combustion zone.

When heated, flame retardants produce:

Free radical traps;

Stabilizing gases;

Diluting gases.

These substances reduce the concentration of free radicals in the flame zone, preventing the combustion chain reaction.

Halogen-containing flame retardants (such as bromine- and chlorine-containing systems) mainly belong to this mechanism.

3.Covering and Isolating Mechanism

Some flame retardants form:

A dense protective layer;

A non-combustible covering film;

A charring barrier.

This protective layer can:

Isolate oxygen;

Reduce the release of combustible gases;

Reduce heat transfer.

For example:

Boron-based flame retardants often function by forming a glassy protective layer.

4.Heat Absorption and Cooling Mechanism

Combustion requires a continuous supply of heat.

Some flame-retardant materials achieve this by:

absorbing a large amount of heat;
decomposing and releasing water of crystallization;

lowering the surface temperature of the material;

thus preventing the spread of combustion.

Typical materials:

Aluminum hydroxide;
Magnesium hydroxide;
Compounds containing water of crystallization.

5.Smoldering Inhibition Mechanism

Smoldering is a significant hazard in the later stages of a fire.

Flame retardants can: inhibit char layer oxidation; reduce further oxidation of CO to CO₂; and reduce sustained heat release.

Phosphorus-containing flame retardants typically exhibit good smoldering resistance.

6.Synergistic Flame Retardant Effect. 
Modern flame retardant systems usually do not rely on a single element but utilize the synergistic effect between different flame retardant elements.

Common flame retardant elements include: phosphorus (P)/nitrogen (N)/bromine (Br)/chlorine (Cl)/boron (B)/antimony (Sb)/sulfur (S)

For example: the phosphorus-nitrogen synergistic system
through: phosphorus promoting dehydration and charring; nitrogen releasing inert gases; and both jointly improving flame retardant efficiency.

IV. Classification of Flame-Retardant Finishes for Textiles

Based on different wash resistance properties, flame-retardant finishes are mainly divided into: non-durable flame-retardant finishes; semi-durable flame-retardant finishes; and durable flame-retardant finishes.

V. Non-Durable Flame-Retardant Finishes

Non-durable flame-retardant finishes are also called temporary flame-retardant finishes.

Characteristics: Simple processing; low cost; minimal impact on hand feel; not washable.

Main applications: Stage curtains; exhibition materials; disposable protective equipment; fabrics that require infrequent washing.

Common flame retardants: Boric acid-borax system; phosphates; ammonium phosphates; metal salt flame retardants.

Finishing can be completed through processes such as padding and drying.

Conclusion

Flame retardant finishing is an important technical means to improve the safety performance of textiles. By selecting a suitable flame retardant system and combining it with scientific finishing processes, the risk of fabric combustion can be effectively reduced, and the added value of products can be increased.

With the continuous development of the industrial protection, fire safety, and functional textile markets, environmentally friendly, efficient, and durable flame retardant finishing technologies will become an important development direction in the future textile auxiliaries field.