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The expansion type water-based fireproof coating relies on a ternary synergistic system of “acid source carbon source gas source” to undergo dehydration, esterification, foaming, and carbonization reactions under high temperature flames, forming a thick, porous, and heat-insulating expansion carbon layer, which is the core material for passive fire protection of steel structures. However, the traditional expansion type fireproof coating system contains a large amount of inorganic flame retardant powder, polyols, polyphosphates and other high filling polarity components. After the paint film is cured, it generally has mechanical defects such as strong rigidity, high brittleness, high internal stress, easy cracking of dry film, poor adhesion, and weak vibration resistance. Under conditions of thick coating during construction, temperature changes in the environment, slight deformation of the substrate, and wind and rain erosion, the fireproof coating is prone to cracking, peeling, peeling, and powdering, leading to premature failure of the fireproof system and inability to form a continuous thermal insulation protective layer.

Introducing suitable polymer toughening agents is the most effective way to improve the flexibility, crack resistance, and durability integrity of fire-resistant coatings by addressing their mechanical brittleness defects. However, unlike ordinary decorative coatings and anti-corrosion coatings, expansion fireproof coatings have a significant balance between mechanical properties and fire resistance: toughening agents are mostly flexible organic polymers, which can improve the toughness of the paint film while easily interfering with the thermal decomposition, melt flow, and foaming carbonization behavior of the ternary flame retardant system, resulting in a decrease in expansion ratio, looseness of the carbon layer, and attenuation of thermal insulation performance. Therefore, the system explores the influence of different types of toughening agents on the mechanical properties, film integrity, thermal expansion behavior, microstructure of carbon layer, and thermal insulation performance of expandable fireproof coatings, clarifies the optimal addition window and performance balance point of toughening modification, and has important engineering value for preparing high-performance expandable fireproof coatings with both high mechanical stability and high fire efficiency.

Intrinsic mechanical defects and toughening modification mechanism of expandable fireproof coating

The solid content of expansive water-based fireproof coatings generally exceeds 70%, and the filling amount of flame retardant powders such as ammonium polyphosphate, melamine, and pentaerythritol in the system is extremely high. The proportion of polymer resin bonding is relatively low. During the process of coating film formation and drying, multiple stresses such as rapid evaporation of moisture, powder accumulation and shrinkage, and resin curing and cross-linking shrinkage are superimposed, resulting in high residual internal stress and numerous micro defects inside the paint film. The dry film of pure fireproof coating exhibits typical brittle material characteristics: extremely low elongation at break, poor impact resistance, easy bending and cracking, strong tendency for thick coating cracking, and continuous crack propagation under long-term natural environment service, ultimately leading to local coating detachment, substrate exposure, and a significant decrease in fire durability.

Toughening agent modification mainly improves the mechanical defects of paint film through three mechanisms: flexible chain segment energy dissipation, microstructure regulation, and stress relaxation. Polyurethane, acrylic ester core-shell, and flexible block toughening agents can construct flexible micro zones inside the paint film. During external tensile, impact, and deformation processes, they absorb fracture energy through elastic deformation, cavitation, and crack deflection, release internal stresses during paint film curing, and significantly improve the elongation at break and crack resistance of the paint film. At the same time, flexible polymer segments can fill the gaps between powder particles, optimize the bonding state of the resin powder interface, reduce pore defects in the paint film, enhance the overall density and adhesion of the coating, and improve the quality of film formation during thick coating construction.

Improvement of Macroscopic Mechanical Properties of Fireproof Coatings by Toughening Agents

Different types of toughening agents can significantly improve the brittle defects of the expansion fireproof coating, effectively enhance the flexibility, impact resistance, and crack resistance of the paint film, and greatly improve the film integrity of the coating under complex working conditions.

Polyurethane tougheners contain a large number of flexible polyethers, polyester soft segments and polar carbamate groups. They have excellent compatibility with fire retardant coatings lotion and flame retardant powders, and can be evenly dispersed in the paint film network, significantly improving the elongation at break and low-temperature crack resistance of the paint film, and solving the problems of low-temperature cracking and temperature difference alternating cracking of fire retardant coatings in winter. At the same time, the polar groups of polyurethane can enhance the interfacial adhesion between resin and inorganic powder, improve coating adhesion and resistance to vibration and detachment, and adapt to steel structure vibration and deformation service scenarios.

Acrylic ester core-shell toughening agent forms nano elastic particles with a soft core and hard shell structure, evenly distributed inside the paint film. When subjected to force, energy is dissipated through particle voiding and matrix shear yielding, and the impact modification effect is outstanding. It can significantly improve the dry shrinkage cracking and surface cracking of thick coated paint films, and enhance the surface smoothness and continuity of the coating.

The experimental results show that within a reasonable range of addition, toughening agents can increase the elongation at break of fireproof coatings by 50% to 120%, significantly improve the cracking resistance level, and increase the single film thickness of thick coatings by more than 30%. This effectively reduces the number of construction passes, improves the efficiency of engineering coating, and significantly reduces the probability of natural aging cracking and peeling of coatings, strengthening the long-term stability of the fireproof system.

But there is a strict threshold for the amount of toughening agent added: at low addition levels, the mechanical properties significantly improve with the increase of toughening agent proportion; When the addition amount exceeds the critical value, the proportion of flexible polymers in the system is too high, and the overall rigidity and hardness of the paint film decrease significantly. The coating may become soft, sticky, have poor scratch resistance, and reduced compressive strength, resulting in an imbalance in the comprehensive mechanical performance and hidden dangers for the deterioration of fire resistance.

The influence of toughening agents on the thermal stability and expansion foaming behavior of coatings

The introduction of toughening agents will significantly change the thermal decomposition law, melt viscosity, and expansion foaming kinetics of the expanded fireproof coating, which is the core cause of the change in fireproof performance. Pure fireproof coating can achieve synchronous matching reactions of “low-temperature dehydration esterification, medium temperature melting foaming, and high-temperature carbon layer solidification” at high temperatures. The ternary system has good synergy, uniform foaming, and high expansion ratio.

Moderate toughening agents can optimize the high-temperature melt rheological properties of coatings, improve the poor melt flowability caused by powder accumulation, and promote uniform gas release and melt expansion during the foaming process, which is beneficial for forming a continuous and uniform expanded carbon layer. At the same time, toughening agents improve the density of the paint film, reduce surface pore defects, and avoid rapid release of flammable gases and insufficient foaming at high temperatures, which have a weak positive promoting effect on expansion behavior.

Excessive toughening agents can have significant negative effects: the thermal decomposition temperature of flexible polymers is generally lower than that of inorganic flame retardant systems, leading to thermal decomposition, weight loss, and carbonization at high temperatures, interfering with the matching rhythm of acid production from acid source decomposition, dehydration and carbonization from carbon source, and gas release from gas source decomposition. On the one hand, polymer pyrolysis consumes system heat and alters local heat conduction rates; On the other hand, excessive flexible melt increases the high-temperature viscosity of the system, hindering gas expansion and stretching, resulting in increased foaming resistance, significantly reduced expansion ratio, and thinner carbon layer thickness, directly weakening the thermal insulation protection ability. In addition, the residual carbon produced by the pyrolysis of toughening agents is mostly loose soft carbon with low mechanical strength and weak resistance to erosion, which is prone to pulverization and detachment under flame erosion, further reducing the fire resistance limit.

The influence of toughening agents on the microstructure and macroscopic fire resistance of carbon layers

The micro pore structure, density, continuity, and mechanical strength of the carbon layer are the core indicators that determine its fire and thermal insulation performance. Toughening agents directly regulate the morphology and structural characteristics of the carbon layer by changing its foaming behavior.

Within the optimal addition range, the carbon layer formed by the combustion of toughened modified fireproof coating has uniform pores, thick pore walls, continuous structure, no through large pores, and no crack collapse. The flexible component optimizes the uniformity of melt foaming, avoiding defects such as uneven foaming, local macropores, and carbon layer fracture in pure systems. The overall density and thermal insulation stability of the carbon layer are higher, which can effectively block heat transfer, oxygen permeation, and substrate heating. The refractory performance is stable and reliable.

When toughening agents are excessively added, the microstructure of the carbon layer undergoes significant deterioration: a large number of large pore sizes, through holes, thin pore walls, loose and fragmented carbon layers appear, the overall mechanical strength of the carbon layer decreases significantly, and it is prone to cracking, collapse, and pulverization under continuous flame burning. Macroscopically, it manifests as insufficient expansion height, loose and non dense carbon layer, accelerated rate of back temperature rise, and significantly reduced fire resistance limit. At the same time, excessive organic components are not fully burned, and a large amount of black carbon and loose carbon deposit remain on the surface of the carbon layer, further reducing the thermal insulation stability and anti-aging performance of the carbon layer.

Performance differences and adaptation rules of different types of toughening agents

There are significant differences in the influence trend of toughening agents with different chemical structures on the mechanical and carbon layer properties of fireproof coatings, and the adaptation scenarios are also different.

Polyurethane toughening agents have the best modification balance, mild toughening effect of flexible chain segments, good compatibility, and minimal interference with ternary flame retardant systems. While significantly improving the anti cracking performance of the paint film, they can basically maintain the original expansion ratio and carbon layer quality, with the highest comprehensive cost-effectiveness, suitable for general steel structure expansion fireproof coatings.

Acrylic core-shell toughening agent has outstanding impact resistance modification, but with high organic carbon content, excessive addition can easily cause the carbon layer to become loose and the expansion ratio to decrease. It is suitable for decorative fireproof coatings that require high surface hardness and flatness of the paint film.

Traditional liquid rubber and flexible small molecule toughening agents have poor compatibility, are prone to migration, and suffer from severe high-temperature pyrolysis interference, which can easily lead to carbon layer collapse and fire failure, making them unsuitable for high-performance expansion fireproof coating systems.

The core performance balance mechanism and technical bottleneck of toughening modification

The toughening modification of expandable fireproof coatings has a natural balance between “mechanical toughness and fireproof performance” recognized by the industry, which is also the biggest technical bottleneck in this field. Toughening agents are essentially flexible organic polymers that enhance the flexibility and crack resistance of paint films, while inevitably changing the organic/inorganic ratio, high-temperature thermal decomposition behavior, and foaming rheological properties of the system. The better the toughening effect of flexible components, the more obvious the interference on high-temperature foaming kinetics, and the more likely it is to cause deterioration of carbon layer quality and decrease in thermal insulation efficiency.

In addition, excessive toughening agents can enhance the overall hydrophilicity of the coating, reduce the water and moisture resistance of the paint film, lead to water absorption softening, decreased adhesion, powdering and peeling of the outdoor service coating, and shorten the durability life of the fireproof coating. How to maximize the retention or even improvement of the thermal insulation and fire resistance of the carbon layer while ensuring excellent mechanical crack resistance is the core difficulty in the development of high-performance and durable expansion fireproof coating formulas.

Performance collaborative optimization strategy

To address the performance balance contradiction in toughening modification, a bidirectional balance between mechanical and fire resistance can be achieved through precise quantity control, structural selection, synergistic compounding, and system optimization.

Firstly, strictly control the addition window of toughening agents and use reactive polyurethane toughening agents with low addition amounts and high toughening efficiency to achieve optimal mechanical improvement with minimal dosage, minimizing the negative impact on the carbon layer structure.

Secondly, selecting high-temperature resistant, low residual carbon, and high thermal stability toughening components to reduce high-temperature pyrolysis interference, maintain the reaction matching of the ternary flame retardant system, and ensure uniform foaming and dense carbon layer.

Thirdly, a composite system of “organic toughening+inorganic reinforcement” should be constructed, utilizing nano silica, kaolin, and functional flame-retardant fillers to enhance the strength of the carbon layer, compensate for the loose defects of the carbon layer caused by organic toughening, and achieve a trinity of anti cracking of the paint film, high strength of the carbon layer, and thermal insulation stability.

Fourthly, optimize the formula acid-base balance and auxiliary system, reduce the interference of dispersants and wetting agents on thermal decomposition behavior, stabilize high-temperature foaming reaction, and ensure the orderly and continuous structure of the expanded carbon layer.

Summary

Toughening agents can effectively solve the mechanical shortcomings of the brittle paint film, easy cracking, weak adhesion, and poor deformation resistance of expansive water-based fireproof coatings. By constructing flexible micro zones, releasing curing internal stresses, optimizing the powder resin interface bonding, the fracture elongation, impact resistance, and film integrity of the coating are significantly improved, and the service durability of the fireproof coating is greatly improved. However, the introduction of toughening agents can alter the high-temperature thermal decomposition and melt foaming behavior of the coating. Excessive addition can lead to a decrease in expansion ratio, loosening and fragmentation of the carbon layer, and degradation of thermal insulation performance, resulting in a significant balance between mechanical and fire resistance properties.

High quality toughening agents such as polyurethane and acrylic core-shell can achieve performance balance within the appropriate addition range, greatly optimizing the mechanical stability of the paint film without significantly damaging the fire resistance. By precise selection, strict quantity control, and organic-inorganic synergistic modification, the performance contradiction of toughening modification can be effectively solved, and high-performance expansion water-based fireproof coatings with excellent anti cracking mechanical properties and efficient and stable fire insulation properties can be obtained, providing technical support for long-term passive fire protection of steel structures.

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