Fireproofing methods for steel structural components primarily include coating protection, fire-resistant board protection, concrete protection, flexible sheet protection, inorganic fiber protection, and internal structural water cooling protection. Among these, applying fire-resistant coatings is convenient, lightweight, cost-effective, and not restricted by the component's geometric shape, offering a wide application range and high efficiency.
To overcome the fire-resistant shortcomings of steel structural materials in actual application, fire protection treatment is necessary, with the aim of raising the fire resistance limit of steel structures to the range specified by the design standards. Measures to prevent the rapid temperature increase and deformation collapse of steel structures in fires are varied, the key being to adopt different methods based on different situations, such as using insulating and refractory materials to block direct flame burning of steel structures, reducing the rate of heat transfer and delaying the time for temperature rise and strength reduction in the steel structures. However, regardless of the method chosen, the principle remains consistent. Fire-resistant coatings are a relatively advanced fire protection technique that has emerged in recent years.
Fire Retardant Coating for Steel Structures: Fire Retardancy Principle and Composition
The principle of fire protection for steel structures involves using insulating or heat-absorbing materials to block direct flame contact with the steel, reducing the rate of heat transfer to the steel and delaying the increase in temperature and the reduction in strength of the steel structure. According to "Fire Retardant Coatings for Steel Structures" (GB14907—2002), fire-retardant coatings for steel structures are defined as coatings applied to the surface of steel structures in buildings and constructions, which can form a fire-resistant and insulating protective layer to enhance the fire resistance limit of the steel structure. Currently, fire-retardant coatings for steel structures, both domestically and internationally, are mainly composed of base resins, catalysts, carbon-forming agents, and foaming agents.
1. Matrix resin
The matrix resin, when combined with other components, ensures that the paint maintains various functional properties under normal conditions and exhibits flame retardant and excellent foaming expansion properties under high temperatures or flame exposure. Generally, the carbonized layer of acrylic resin fire-resistant paint is of high quality, which is why acrylic resin is commonly used as the primary film former and is modified to enhance the overall performance of the paint.
2. Catalyst
A catalyst is a substance that can decompose into phosphoric acid under certain conditions. The acid produced dehydrates polyhydric alcohols, thereby forming a non-flammable carbonized layer with a three-dimensional spatial structure. Generally, the water solubility of melamine phosphate is lower than that of phosphoric acid amine, and it also has both catalytic and foaming effects. Currently, melamine phosphate is primarily chosen as the catalyst.
3. Carbon Agent
Carbon agents are the fundamental substances that form a non-flammable, three-dimensional foam carbonization layer on the coating under high temperatures, serving as the skeletal structure for the foam carbonization layer. The carbon agents must match the decomposition temperature with the catalyst; when polyphosphoric amine is used as the catalyst, high-temperature stability carbon-rich polyhydroxy compounds, such as pentaerythritol and dipentaerythritol, should be used as the carbon agents. Super-thin flame-retardant coatings using these agents have been used on the exposed factory buildings of the Daya Bay Nuclear Power Station in Guangdong for over a decade and are still in normal use. However, their drawbacks include strong odor during application and easy aging of the coating. Issues with water resistance in coatings made with starch are not easily resolved, and dipentaerythritol is rarely used domestically due to its price. Currently, pentaerythritol is widely used as the carbon agent in flame-retardant coatings in China.
4. Foaming Agent
Expanding fire-resistant coatings can only produce an expanded layer under the action of a foaming agent in the presence of high-temperature flames. The foaming agent decomposes upon contact with fire, releasing non-flammable gases such as ammonia, water, carbon dioxide, and hydrogen halides. This causes the coating to foam and expand upon reaching its softening point, forming a spongy structure.