Flame retardancy is the process of making flammable materials such as wood and fiber less flammable, thereby preventing the outbreak of fire. Here we will provide a simple explanation of flame retardant treatment using flame retardants.

1. The beginning of fire retardancy

Human beings have created civilization by discovering and utilizing "fire." Since the discovery of oil, large amounts of "thermal power" have spurred the development of civilization and brought us the "affluent" lifestyles of the 20th century.
However, at the same time, it is also true that fire, which brought civilization to mankind, has caused various disasters and taken many irreplaceable lives and property.
For this reason, humans have continued to seek ways to make their everyday items safer since the dawn of history, and have long attempted to research flame retardancy, in other words, ways to limit or slow the spread of fire, by making textiles, paper products, wood products, and more recently plastic products, which are often exposed to fire in daily life.
According to historical records, the beginning of flame-retardant textile processing dates back to the mid-17th century, when playhouse tents were treated with clay and plaster. Throughout the 19th century, flame-retardant processing mainly involved "inorganic salts," and today it has become an inseparable part of our daily lives.

2. The concept of flame retardant processing

Generally, when a substance burns, the phenomenon can be divided into two categories.

• 1. A phenomenon in which a substance changes and decomposes due to heat, generating flammable gases, which then burst into flames and burn.
• ② The phenomenon in which carbonized residue oxidizes and gradually burns.

The former is called "combustion" or "flaming" and the latter "jin-yaki" or "glowing." The word "flame proofing" includes the two meanings of "flame proofing" and "glow proofing."
In our daily lives, we need flame retardant

• ① Cellulose and natural fibers, paper
• ② Synthetic fibers and synthetic resins
• ③ Wood and plywood

can be roughly divided into

According to statistics in the United States, approximately 20% of fire accidents are caused by textile products, making it clear that flame-retardant processing of textile products is particularly necessary.

3. Requirements for flame retardants

• 1. A phenomenon in which a substance changes and decomposes due to heat, generating flammable gases, which then burst into flames and burn.
• ② It must not emit any toxic gases or smoke when burned.
• ③ Processing can be done using facilities and machinery that are available in general processing plants.
• ④ The properties of the fabric and other materials must not be impaired (touch, tear strength, tensile strength, etc.)
• ⑤ Durability against washing, etc.
• ⑥ Do not generate any powdery substances on the surface.
• ⑦ The material must not absorb moisture beyond a certain level and must not become damp or sticky.
• ⑧ When burned, it forms a strong sap that remains as a fabric between the outside air and the skin.
• ⑨ To provide fire resistance as well as dust resistance
• 10. Does not interfere with post-processing of textiles, such as texture adjustment, dyeing, waterproofing, and resin finishing.

4. Flame retardant mechanism

• ① Endothermic effect: When flame retardants decompose at high temperatures, they absorb a large amount of heat and self-extinguish.
• ② Coating: A glassy charcoal layer is formed on each material, blocking the material from air (oxygen).
• ③ Gas dilution: When flame-retardant materials are heated, they generate non-flammable gases, diluting flammable gases and preventing them from igniting.
• ④ Changes in pyrolysis reaction: Cross-linking and hydrogen bonding between flame retardants and materials, increasing the production of persistent substances and crystalline carbon.
• ⑤ Dehydration reaction: When heated, it promotes the decomposition of cellulose into crystalline carbon and a large amount of water.

5. Representative examples of flame retardant mechanisms

Synthetic Fibers

Unlike natural fibers, when it comes into contact with fire, it first shrinks and melts into balls, making it difficult to burn, but once it starts to burn, it burns violently. Although it is impossible to give a uniform explanation, the effective flame retardant properties are as follows:

• ① The non-flammable gas produced by pyrolysis dilutes the flammable gas.
• ② Create a non-flammable substance to break the chain reaction
• ③ Synthetic fibers generate flammable radicals through thermal decomposition, but the chain reaction of decomposition can be stopped by converting the HO・ generated into H₂O.

●Decomposition process of synthetic fibers and synthetic resins

●Fire retardant mechanism

6. Timber and plywood

Pyrolysis of wood begins at around 200°C, producing a large amount of combustible materials with low ignition points, which burn explosively.
Therefore, the flame retardant mechanism must have a synergistic effect between physical and chemical actions.

• ① Coating effect that prevents contact with oxygen in the air
• ② Forming a layer of non-flammable gas to reflect heat from the outside or prevent it from being transmitted to the inside
• 3) Chemical action that promotes the formation of a charred layer that is effective in fire prevention
• (a) Endothermic reactions suppress heat and prevent chain reactions.
• (Brief) Produces non-flammable gas and dilutes flammable gas
• (c) It promotes carbonization and graphitization, and the tough carbonized layer that is formed does not allow flames to penetrate to the back side.
• (D) The carbonized layer acts as a heat insulator.

The dust-proofing effect is thought to be the same as that of fiber.

7. Dust prevention effect

The combustion of charcoaled fibers and other materials is considered to be the direct combustion of carbon, like coke or charcoal.
In this case, the burning carbon is oxidized in one of two ways depending on the surrounding conditions:

• (a) C + 1/O26 → CO + 1 kcal/mol [First-order reaction] ⇒CO + 68/OXNUMX → COXNUMX + XNUMX kcal/mol [Second-order reaction]
• (b) C + O94 → COXNUMX + XNUMX kcal/mol

"Dust prevention effect" refers to preventing the primary reaction (a) in which a solid is directly oxidized, and the reaction (b).

• (1) When decomposing, heat is absorbed, suppressing the reaction.
• (2) Non-flammable gas prevents carbon from coming into contact with air (oxygen)

These are its two main effects.

* In the secondary reaction of (a), gas burns, so it results in flame burning rather than dust burning. If the secondary reaction stops, the heat generated by the primary reaction is insufficient, and without additional heating, the reaction will slow down rapidly and combustion will tend to stop naturally.

What is important here is that when flame-retardant treated fibers burn, they first decompose and emit flammable gases, and the flame retardant contained in the fibers exerts its effect, but at this time, the flame retardant must not be entirely dispersed but remain in the carbonized residue. If it were to completely decompose, the tarred portion would not be flame-retardant treated and would be relatively easy to burn. If a certain amount of flame retardant remains after carbonization, it will further decompose during the oxidation reactions (a) and (b) above, and exert the dust-suppressing effect as shown in (1) and (2).
Furthermore, crystalline carbon is much less flammable than amorphous carbon, so when fibers and other materials decompose due to heat, it is thought that this promotes the crystallization of carbon, thereby providing excellent dust-proofing effects.

8. Fire-retardant terminology

This is an explanation of the terms used on this page.