How Does Fire-Retardant Wood Board Achieve Fire Resistance?

How Does Fire-Retardant Wood Board Achieve Fire Resistance?

How does fire-retardant wood board achieve fire resistance? Wood is one of the world's four major materials, prized for its natural and beautiful grain, environmental friendliness, and non-toxic properties. It is widely used in construction, furniture, and other fields. However, because wood materials are flammable and can easily lead to fires, threatening lives and property, research into the fire-retardant properties of wood is imperative. This article details how fire-retardant wood boards achieve fire resistance.

I. Theory of Wood Flame Retardancy
Wood is a porous, complex natural organic material composed of cellulose, hemicellulose, and lignin. Its high hydrocarbon content makes it a flammable material. Wood flame retardancy involves improving the wood's resistance to combustion through physical or chemical methods, aiming to slow down burning and prevent fire incidents. The goal of wood flame retardancy is to reduce the burning rate, decrease flame spread speed, and accelerate the surface carbonization process during combustion.
Currently, an effective measure for making wood boards flame-retardant is adding flame retardants. The working principles of flame retardants can be broadly categorized into the Covering Theory, Thermal Theory, and Non-Combustible Gas Dilution Theory.

1. Covering Theory: This posits that the flame retardant melts at a temperature lower than the wood's ignition point, forming an insulating layer. This layer isolates the wood from the flame and prevents the escape of combustible gases, thus achieving a flame-retardant effect.

2. Thermal Theory: This suggests that the flame retardant absorbs a significant amount of heat during its decomposition process, delaying the temperature rise of the wood and thereby suppressing surface ignition.

3. Non-Combustible Gas Dilution Theory: This theory states that the flame retardant decomposes when heated below the wood's normal combustion temperature, releasing non-combustible gases or water vapor. These dilute the concentration of combustible gases produced by the thermal decomposition of the wood, contributing to flame retardancy.

II. Wood Board Flame Retardants

1. Inorganic Flame Retardants

Inorganic flame retardants primarily include elements like phosphorus, nitrogen, boron, aluminum, and halogens. They are characterized by wide availability, low cost, environmental safety, and good flame retardant performance, making them the most widely used type currently.

Borates, such as boric acid and borax, are the most common boron-based flame retardants. They exhibit low toxicity to humans, are environmentally friendly, and effectively reduce flame spread.

Commonly used phosphorus-nitrogen flame retardants include inorganic phosphates like ammonium polyphosphate and melamine phosphate.

Metal hydroxide flame retardants, such as aluminum hydroxide and magnesium hydroxide, do not produce toxic or corrosive gases during the flame retardancy process, making them environmentally friendly green flame retardants.

2. Organic Flame Retardants
Inorganic flame retardants have drawbacks like strong moisture absorption and poor retention. Organic flame retardants can overcome these issues. Building on this, researchers have developed organic flame retardants, mainly including organophosphorus-nitrogen, organophosphorus-boron types, as well as resin-based organic flame retardants made from raw materials like formaldehyde, urea, sodium hydroxide, dicyandiamide, and ammonium dihydrogen phosphate. However, organic flame retardants have disadvantages such as higher production costs and less stable performance, making them less commonly used in practical applications compared to inorganic flame retardants.

III. Wood Board Flame Retardant Treatment Technologies
The flame retardant performance of wood boards depends not only on the properties and quantity of the flame retardant but is also closely related to its distribution within the wood. The treatment process affects the wood's moisture absorption and flame retardancy. Selecting an appropriate treatment process is crucial to avoid compromising the wood's physical and mechanical properties.

1. Impregnation Method
The impregnation method involves introducing flame retardants into wood under conditions of normal pressure, vacuum, pressure, or a combination thereof.

• Normal Pressure Impregnation: This is conducted under atmospheric pressure. The process is relatively simple, low-cost, and requires less equipment investment. However, it is only suitable for thin materials like veneers or materials with good permeability.

• Pressure Impregnation: This method places wood in a high-pressure vessel. A vacuum is first applied to remove gases from within the wood, drawing in the flame retardant solution upon release of the vacuum. Pressure is then applied to force the retardant deep into the wood.

• Stepwise Impregnation: This method involves impregnating with different flame retardants sequentially, allowing the chemicals from preceding and subsequent treatments to react with each other, forming precipitates. Wood treated this way can achieve a weight gain of over 20% after drying. Furthermore, the dried wood shows significant improvements in properties like ceramification, flame retardancy, hardness, and dimensional stability.

2. Coating/Spraying Method
For wood that is not suitable for impregnation, methods like brushing or spraying flame retardants can be used. This isolates the heat source and prevents the wood from contacting air, thereby reducing its combustibility.

3. Hot Pressing Method
The hot pressing method involves sprinkling flame retardant powder onto the board surface or brushing on a retardant solution. Under heat and pressure, the retardant melts and penetrates into the wood. This method avoids surface swelling during pressure impregnation and the subsequent drying step. Its disadvantage is the difficulty in applying a sufficient dosage.

Conclusion

With the dual-standard upgrade of "flame retardancy + environmental protection" for furniture wood boards, inorganic phosphorus-nitrogen-boron systems, while mainstream, are just the beginning.

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