The application thickness of intumescent fire retardant coating is a key parameter determining its fireproofing performance. Its influence spans the entire process of thermal expansion, thermal insulation, and structural stability. Application thickness directly determines the coating's expansion ratio and the quality of the char layer during a fire. When heated, the coating's internal expanding components (such as ammonium polyphosphate and melamine) decompose to produce non-combustible gases, causing the coating to expand and form a porous char layer. If the application thickness is insufficient, the char layer will be too thin after expansion, failing to effectively block heat transfer and causing the substrate to heat up rapidly. Excessive thickness can lead to cracks or even detachment of the char layer due to uneven expansion or increased weight, ultimately reducing the fireproofing effect. Therefore, properly controlling the application thickness is crucial to ensuring the integrity and thermal insulation performance of the char layer.
The fire resistance limit of intumescent fire retardant coating exhibits a nonlinear relationship with application thickness. Experiments show that before the coating reaches a critical thickness, the fire resistance time increases significantly with increasing thickness; however, after exceeding this critical value, the fire resistance time slows or even decreases. For example, a certain ultra-thin intumescent fire retardant coating has a fire resistance limit of 0.5 hours at a thickness of 1.5mm, increasing to 1.5 hours at a thickness of 3mm. However, when the thickness is increased to 5mm, the fire resistance limit only extends to 1.8 hours. This phenomenon occurs when an overly thick coating expands, resulting in uneven internal stress distribution and cracking of the carbonized layer. Furthermore, the increased weight increases the risk of delamination between the coating and the substrate, thereby weakening the thermal insulation effect.
Application thickness also significantly affects coating adhesion and seismic performance. Intumescent fire retardant coatings require strong adhesion to ensure a tight bond with the substrate during a fire to prevent flaking due to expansion stress. Excessive coating thickness can lead to uneven drying and shrinkage within the coating, which can reduce adhesion. Furthermore, overly thick coatings are more susceptible to cracking due to substrate vibration or thermal expansion and contraction, compromising long-term protection. Therefore, the appropriate coating thickness should be selected based on the substrate type (e.g., steel structures, concrete). For example, steel structures typically require a coating thickness of 2-5mm to achieve a balance between adhesion and fire resistance.
The application thickness of intumescent fire retardant coatings also impacts their economical efficiency and application efficiency. Ultra-thin coatings (thickness ≤ 3mm) offer lower overall costs due to fewer layers and faster drying. Thin coatings (3-7mm), while requiring multiple coats, use less per unit area, keeping material costs manageable. Blindly increasing the coating thickness to achieve excessively high fire resistance not only increases material consumption and labor costs, but can also prolong drying times and even cause defects such as sagging and blistering, ultimately reducing application quality.
Environmental factors and application conditions place higher demands on thickness control. In low temperatures or high humidity, coating drying slows, necessitating adjustments to the coating thickness or increased drying time to ensure coating performance. When applying to vertical surfaces or ceilings, excessively thick coatings can easily flow due to gravity, necessitating layered spraying or the addition of thickeners (such as sodium carboxymethyl cellulose) to maintain coating stability. Furthermore, the choice of application tools (such as spray gun pressure and brush hardness) can also affect coating uniformity and should be tailored to the desired coating viscosity and thickness.
The application thickness of intumescent fire retardant coating must be precisely matched to the fire resistance design requirements. Design specifications typically specify minimum coating thickness based on the building structure type, fire hazard level, and fire resistance limit requirements. For example, the steel beams of a large commercial complex must meet a 2-hour fire resistance limit, and calculations dictate a 4.5mm thick coating. Failure to achieve a sufficient coating thickness will directly lead to fire protection failure and threaten building safety. Therefore, pre-application testing is necessary to verify the relationship between coating thickness and fire resistance performance to ensure compliance with design standards.
The application thickness of intumescent fire retardant coating is a multi-dimensional variable that influences its fire resistance, economic efficiency, and construction feasibility. Appropriate thickness control requires comprehensive consideration of material properties, substrate type, environmental conditions, and design requirements. A scientifically formulated coating and meticulous application process are crucial to achieve the optimal balance between fire protection effectiveness and cost-effectiveness.
