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Development Trends and Functions of Fire-Resistant Coatings Authors: Wang Ying, Ji Xiaoming, Chen Lanpin, Zhu Rongyou In light of the widespread use of steel structures in construction projects in recent years, and taking into account the characteristics and hazards associated with fire risks, this paper discusses the performance and applicability of several types of fire-resistant coatings, and outlines the future development directions for such coatings. 0 Introduction Building fire protection is an important field within fire science and technology, and fire-retardant coatings are a crucial component of fire-resistant building materials. Fireproof coatings are types of coatings applied to the surface of objects to prevent fires from occurring, stop the spread of flames, or isolate the fire source; they also serve to prolong the time before the substrate catches fire or enhance insulation properties in order to delay structural damage. Based on their purpose and intended use, they can be classified into: decorative fireproof coatings, cable fireproof coatings, fireproof coatings for steel structures, fireproof coatings for prestressed concrete floors, etc. The development of fire-resistant coatings in our country started 15 to 20 years later than in industrially advanced countries abroad; although it began late, its growth rate has been fast. Especially in terms of fire-resistant coatings for steel structures, they have approached or reached international advanced levels in terms of variety, technical performance, application effectiveness, and standardization. In recent years, its demand has grown exponentially, which is closely linked to the rapid development of steel structure buildings. 1 Fire-retardant coatings for steel structures: As a type of structure used in high-rise buildings, steel structures are widely employed in the construction industry due to their high strength, light weight, good ductility, seismic resistance, and short construction time. They exhibit particular versatility in ultra-high-rise and large-span buildings. By the end of 1990, steel structures accounted for 79% of the 100 super-tall buildings in the world that were over 200 meters high. Since the reform and opening up, with the strengthening of international technological exchanges and cooperation, the application technology of steel structures has seen rapid development in China, leading to a swift increase in high-rise and super-high-rise buildings. According to statistics from 1995, steel structures accounted for 37% of the 100 super-tall buildings constructed in China, with most of them having been built in recent years. With the growth of urban sizes in our country, the application of steel structures in the construction industry holds very broad prospects. However, since steel structures are non-flammable in themselves, the issue of fire and heat insulation protection for such structures was once overlooked. According to domestic and international reports, as well as the tests and statistical data from relevant institutions, the fire resistance of steel structure buildings is inferior to that of masonry and reinforced concrete structures. The mechanical strength of steel decreases as the temperature rises; at around 5,000 °C, its strength drops to 40%–50%. The mechanical properties of steel, such as yield strength, compressive strength, elastic modulus, and load-bearing capacity, all decline rapidly, causing it to lose its ability to provide support and leading to the collapse of buildings. Therefore, it is imperative to protect steel structures. Fire-resistant coatings for steel structures are applied to the surface of these structures by brushing or spraying; they serve to prevent fire and heat from affecting the steel, stopping it from heating up rapidly and losing its strength. This prevents the steel structure from losing its supporting capacity and causing the building to collapse. As early as the 1970s, foreign countries carried out active research and application work on fire-resistant coatings for steel structures, achieving good results, and this field remains dynamic to this day. In the early 1980s, foreign fire-resistant coatings for steel structures entered the Chinese market and were used in construction projects. Since the early 1980s, China has also begun to develop fire-retardant coatings for steel structures, and to date many high-quality varieties of such coatings are widely used in various industries. 2 Thick-coated fire-resistant coatings for steel structures: Thick-coated fire-resistant coatings for steel structures refer to coatings with a thickness of 8 to 50 mm; such coatings can provide a fire resistance duration of 0.5 to 3 hours. In the event of a fire, the coating does not expand; by virtue of the material’s non-flammability, low thermal conductivity, or the heat-absorbing properties of the materials in the coating, it slows down the heating of the steel, thereby protecting the steel components. Such fire-resistant coatings for steel structures use appropriate binders, along with inorganic lightweight materials and reinforcing materials. Compared with other types of fire-retardant coatings for steel structures, in addition to possessing some of the advantages of water-soluble fire-retardant coatings, it has a low cost because both its base material and most of its additives are inorganic substances. The application of fire-resistant coatings for such steel structures is generally done by spraying, and they are often used on indoor steel structures where a fire resistance rating of over 2 hours is required. However, due to their thick coating, such products have relatively poor aesthetic appeal. 3 Thin-coat type fire-resistant coating for steel structures: Fire-resistant coatings for steel structures with a coating thickness of 3 to 7 mm are referred to as thin-coat type fire-resistant coatings for steel structures. When exposed to fire, this type of coating expands and forms bubbles; the fire-resistant insulating layer created by this expansion helps to slow down the heating of the steel, thereby protecting the steel components. Such steel structure coatings generally use suitable latex poly__ compounds as the base, along with flame retardants, additives, and other components. For such fire-resistant coatings, it is required that the emulsion polymer used must have good adhesion, durability, and water resistance to steel substrates. Emulsion polymers commonly used as binders for such fire-resistant coatings include styrene-modified acrylic emulsions, polyvinyl acetate emulsions, vinylidene chloride emulsions, and others. In fire-retardant coatings based on aqueous emulsions, flame-retardant additives, pigments, and fillers are dispersed in water; thus, water acts as the dispersion medium. To enable better dispersion of these granular additives, dispersants such as sodium hexametaphosphate, which is commonly used, are also added. The production of such fire-resistant coatings for steel structures generally involves 3 steps: in the first step, various flame-retardant additives are dispersed in water, and then ground into a slurry of the desired fineness ； In the second step, the paint is prepared using the base material (emulsion) ； In the third step, inorganic lightweight materials and reinforcing materials are added to the slurry and mixed thoroughly. This coating is generally divided into a base layer (insulation layer) and a top layer (decorative layer); it has better decorative properties than thick-coat types. It is applied by spraying and is typically used on building steel structures where the required fire resistance duration is no more than 2 hours. 4 Ultra-thin fire-resistant coating for steel structures: An ultra-thin fire-resistant coating for steel structures refers to a coating with a thickness of no more than 3 mm. When exposed to fire, such coatings expand and foam, forming a dense fire-resistant and insulating layer; it is a new type of coating that has been developed in recent years. It can be applied by spraying, brushing, or rolling, and is generally used on building steel structures where a fire resistance rating of up to 2 hours is required. Compared with thick-coated and thin-coated fireproof coatings for steel structures, ultra-thin expansion fireproof coatings for steel structures have lower viscosity, thinner coats, are easier to apply, and offer better decorative properties. These coatings are highly favored by users as fire-resistant coatings for steel structures, as they meet fire safety requirements while also fulfilling high aesthetic standards, especially for exposed steel structures. The Fire Research Institute of the Ministry of Public Security has developed ultra-thin intumescent fire-resistant coatings for steel structures, namely “SCB” (solvent-based) and “SCA” (water-based). The coating thicknesses of these coatings are 2.69 mm and 1.6 mm respectively, with fire resistance times of 147 minutes and 63 minutes respectively. Another set of ultra-thin fire-resistant coatings for steel structures is “LF” (solvent-based) and “L6” (solvent-based); their coating thicknesses are 2 mm and 3 mm respectively, with fire resistance times of 94 minutes and 90 minutes respectively. The “Watet Base” 38320 type fire-resistant coating for steel structures, produced by the German company Herberts (water-based), has a coating thickness of 2.63 mm and a fire resistance time of 63 minutes. The 38091 type fire-resistant coating for steel structures is solvent-based, with a coating thickness of 2.42 mm and a fire resistance time of 124 minutes. The British company Nullifire produces fire-resistant coatings for steel structures that are solvent-based; their coating thickness is 2.24 mm, with a fire resistance time of 106 minutes. Jiangsu Lanling Company offers ultra-thin fire-resistant coatings for steel structures, namely “SF” (solvent-based) and “ECB” (water-based). The coating thicknesses of these coatings are 2.07 mm and 1.6 mm respectively, with fire resistance times of 150 minutes and 44 minutes respectively. In summary, since research on fire-resistant coatings for ultra-thin steel structures in China has been ongoing for only a short time, progress has been rapid in studying the fire-resistant and physicochemical properties of these coatings. However, to develop ultra-thin fire-resistant coatings suitable for outdoor use with excellent performance, further research is needed regarding their weather resistance. Foreign steel structure fireproof coatings are evolving toward being ultra-thin, possessing excellent weather resistance, and having good decorative properties. Weather resistance tests have been conducted in accordance with European aging test standards, and these coatings exhibit outstanding weather resistance; their film’s weather resistance meets the requirements for outdoor use. Therefore, in order to keep up with or surpass similar foreign products and meet market demands, the development of high-weather-resistant ultra-thin outdoor steel structure fireproof coatings is the direction for future progress. 5 Decorative fireproof coatings In addition to fireproof coatings for steel structures, decorative fireproof coatings and cable fireproof coatings have also seen rapid development. Finish-type fireproof coating is a new type of coating that combines decorative functions with fireproofing properties; when applied to combustible substrates, it can serve as a decorative element in normal conditions ； Once a fire breaks out, it prevents the spread of flames, thereby protecting the combustible materials. It is precisely due to this special purpose that, in countries with advanced industries, fire-resistant coatings appeared as early as the 1920s. The development of this coating has gone through two stages. Initially, inorganic fireproof coatings using silicate salt glass as a binder were used. Such coatings do not burn on their own; when exposed to fire, they form a hollow foam layer that provides some protection for the combustible substrate. However, their insulation properties and weather resistance are poor, and they tend to turn white, crack, and peel off. In the late 1940s, efforts began to develop organic intumescent fire-retardant coatings. The main advantage of these coatings is that they offer better fire-retardant properties as well as superior physical and chemical characteristics compared to inorganic fire-retardant coatings. When exposed to fire, such coatings form a dense, sponge-like intumescent foam layer with excellent heat-insulating properties, thereby providing more effective protection for combustible substrates. Compared to inorganic fireproof coatings, organic intumescent fireproof coatings represent a more promising type of fireproof coating, and as such they experienced rapid development over the following decades. By the end of the 1970s, the annual global sales volume of intumescent fireproof coatings had exceeded 7,380 tons. The development of decorative fireproof coatings in our country has also gone through two stages. The fire-resistant coatings that appeared in the late 1950s were also inorganic fire-resistant coatings using silicate salt glass as a binder. In the early 1970s, some specialized paint manufacturers produced perchloroethylene and chlorinated rubber fire-retardant paints. Due to their unsatisfactory fire-resistant performance, the above two types of fireproof coatings have not established a market in China. It was not until the late 1970s that our country began research on organic intumescent fire retardant coatings. The earliest expansion-type fire retardant coatings were developed by the Sichuan Fire Science Research Institute of the Ministry of Public Security: the B60-1 acrylic expansion fire retardant coating and the A60-1 modified amino expansion fire retardant coating. Since then, organic intumescent fireproof coatings in our country have developed rapidly. To date, the number of fireproof coating manufacturers in our country has grown to over 200, with sales amountsing to tens of thousands of tons; a cross-departmental research and production system has been established. Finish-type intumescent fireproof coatings can be divided into solvent-based and water-based types. The fireproofing components used in these two types of coatings are essentially the same, so it is difficult to say that there is a significant difference in their fireproofing performance. The solvent chosen depends on the film-forming substance used. The film-forming substances used in solvent-based fireproof coatings are generally rubber chlorides, perchloroethylene, amino resins, phenolic resins, etc., while the solvents employed include No. 200 solvent gasoline, spray paint thinners, butyl acetate, and others. The film-forming substances in water-based fireproof coatings are generally vinyl chloride-vinylidene chloride emulsions, styrene-acrylic emulsions, pure acrylic emulsions, polyvinyl acetate emulsions, etc.; all of these materials use water as a solvent. The main difference in the performance of these two types of coatings lies in their physical and chemical properties as well as their weather resistance; solvent-based fireproof coatings outperform water-based fireproof coatings in both of these aspects. Transparent fireproof coatings are a type of decorative fireproof coating that has been developed in recent years and has become increasingly mature. These coatings are widely used in the decoration of wooden structures in hotels, hospitals, theaters, computer rooms, as well as for the decorative and fireproofing purposes of various high-rise buildings and historic structures. However, with the rapid development of China’s industry and the demands in the market, higher requirements have been placed on transparent fireproof coatings. These coatings are not only expected to have good fireproofing properties, but also to possess a transparent and shiny finish as well as excellent weather resistance. 6 Cable fire-retardant coatings: The development of cable fire-retardant coating products in China began in the late 1970s and early 1980s. These coatings were developed based on decorative fire-retardant coatings, taking into account specific requirements. They possess good physical and chemical properties as well as weather resistance; the coating layer is thin, and when exposed to fire, it forms a uniform and dense sponge-like foam insulation layer that provides significant thermal and fire-retardant effects. This helps to protect cables, prevent the spread of flames, and stop the occurrence and progression of fires. As an important product for the fire protection of cables, cable fire-resistant coatings have, through their use over the past 20 years, played a positive role in reducing losses caused by cable fires and protecting people’s property. Their application has also evolved from being irregular to becoming more standardized. However, due to the rapid development of modern society and the diversity in the environments in which cables are used as well as the ways in which they are installed, based on years of experience with cable fireproof coatings, it can be seen that current water-based fireproof coatings still require improvements in certain aspects of their performance in order to meet the requirements of the environments in which cables are used. Solvent-based fireproof coatings are currently the ones that are used quite frequently. However, since these coatings are flammable in nature, they pose significant fire hazards. Moreover, solvents can cause varying degrees of harm to the human body; therefore, extra safety measures must be taken when using them in spaces that are narrow or have poor ventilation, such as cable shafts, cable trenches, and cable tunnels. From an environmental perspective, efforts should be made in the future to develop water-based fireproof coatings with excellent physical and chemical properties as well as weather resistance. 7 Fire-resistant coatings for prestressed concrete floors: In the rapidly developing construction industry, concrete-based building structures are very common. Prestressed reinforced concrete offers better crack resistance, stiffness, shear strength, and stability compared to ordinary reinforced concrete; it is also lighter in weight, allowing for savings in concrete and steel usage. However, prestressed reinforced concrete hollow floor slabs, which are widely used, have poor fire resistance. The reason for this is that when the temperature of the prestressed rebar reaches 2000 °C, its yield point begins to decrease; at 3000 °C, the prestress almost disappears and creep accelerates, resulting in a rapid decline in the strength and stiffness of the slab. This in turn leads to an increased deflection of the slab, cracks appearing beneath it. The prestressed rebar, being exposed to high temperatures, experiences further reductions in stiffness and strength. The properties of the concrete also change under high temperatures, and the thermal expansion of the concrete beneath the slab occurs in the same direction as the tensile force acting on the slab, thereby exacerbating the deflection of the slab. At 3000 ℃, the strength of concrete begins to decline; at 5000 ℃, its strength is reduced by about half, and at 8000 ℃, its strength is almost completely lost. In building fires, these types of floors collapse within about 0.5 hours. Prestressed concrete hollow slabs are widely used in modern buildings as load-bearing floor slabs. However, their poor fire resistance poses a challenge to complying with building design fire safety codes. To improve the fire resistance limit of prestressed floor slabs, people first tried increasing the thickness of the reinforced concrete cover layer, but the effect was not significant; instead, it increased the weight of the slab and occupied useful space. Drawing on the principle of using fire-resistant coatings on steel structures to protect them, China began to research and produce fire-resistant coatings for prestressed concrete floors in the mid-1980s. These coatings are widely used to protect prestressed floors; they are applied to the side of the floor that contains the reinforcement. In the event of a fire, these coatings effectively prevent flames and heat from reaching the concrete as well as the prestressed rebar within it, thereby reducing the rate at which heat is transmitted and delaying the rise in temperature. This helps to increase the fire resistance of prestressed floors and achieve fire protection purposes. 8 Conclusion As efforts to develop urban areas in our country continue to increase, steel structures will be widely used in large-scale exhibitions, sports centers, and high-rise buildings, thanks to their excellent strength and ductility. Therefore, it is necessary to conduct further in-depth research on the performance of fire-resistant coatings for steel structures. By meeting various needs and functions, it is possible to increase the fire resistance of steel structures, enhance a building’s ability to withstand fires, and ensure the safety of lives and property.