Fire-retardant properties of wooden fire doors are closely tied to their fire-resistance rating. As a result, many manufacturers designate the flame-retardant process as a critical control point in production. However, due to the lack of standardized procedures for this step, each manufacturer uses different techniques, often leading to inconsistent and unreliable flame-retardant performance. In extreme cases, this can reduce the fire resistance of the door, posing serious risks to life and property.
Common methods for treating wood with flame retardants include spraying, soaking, steaming, vacuum impregnation, and others. While spraying and immersion are typically used on surfaces that won’t be planed or on thin sheets (under 10 mm), they are not suitable for manufacturing wooden fire doors. Vacuum and pressurized methods offer better results but are rarely used by local manufacturers due to high equipment costs. Most companies in our region currently use steam treatment. The author suggests several key considerations when applying this method:
First, the moisture content of the wood plays a crucial role in the effectiveness of flame retardant treatment. Dry wood tends to absorb more flame-retardant liquid, which directly affects the oxygen index and depth of fire resistance. The higher the absorption, the better the flame resistance. However, many companies overlook this factor, using wet or freshly cut wood without drying it first. This leads to unstable results. Experiments show that wood with less than 15% moisture can increase its oxygen index by over 5 points and achieve a flame-retardant depth of more than 3 mm, compared to wood with over 40% moisture.
Second, the sequence of the flame-retardant process matters. Many manufacturers follow a pattern of wood → flame retardant → water → start, which can lead to the wood absorbing low-concentration solutions. This affects the absorption rate and dry salt content, reducing the overall effectiveness. A better approach is to prepare a pre-mixed flame-retardant solution before starting, ensuring the wood absorbs the correct concentration from the beginning. This helps maintain consistent quality and improves the final outcome.
Third, controlling evaporation during the process is essential. Traditional methods involve open heating, causing rapid evaporation of the flame-retardant solution. This leads to fluctuating concentrations, requiring constant water addition, which dilutes the solution and reduces its effectiveness. To avoid this, companies should minimize water addition during treatment and maintain a stable concentration. If water must be added, it should be done carefully to preserve the integrity of the flame-retardant solution.
Lastly, allowing the wood to cool at room temperature after treatment can enhance flame-retardant performance. Many manufacturers discharge the wood immediately after the required time, without considering the cooling phase. During cooling, thermal expansion and contraction create negative pressure inside the wood, helping it absorb more flame retardant. This additional absorption improves the overall quality of the treatment and ensures better long-term performance.