Expansion type water-based fireproof coating is an important passive fire protection material for steel structures and building components. It relies on foaming expansion at high temperatures and carbon layer insulation mechanism to delay substrate temperature rise and improve the fire resistance limit of components. A typical expansion type fireproof coating is a high solid content, high color filler, multi salt system, thick coating water-based system. The formula contains a large amount of expansion flame retardant components such as ammonium polyphosphate, pentaerythritol, melamine, and inorganic powder fillers. The solid content of the system is generally higher than 70%, and the pigment volume concentration is extremely high. The difficulty of rheological control is much greater than that of ordinary water-based building coatings and conventional industrial paints. Although traditional cellulose ethers, alkali swelling thickeners, and bentonite systems have significant thickening effects, they suffer from problems such as uneven leveling, cracking of thick coatings, uneven foaming, loose carbon layers, and decreased adhesion, which seriously affect fire and thermal insulation performance. Hydrophobic modified polyurethane associative thickener (HEUR) is gradually being applied to high-performance expansion water-based fireproof coatings due to its reversible hydrophobic associative rheological properties, significant shear thinning, good thick coating stability, and minimal negative impact on film density. This article systematically elaborates on the rheological control mechanism of HEUR thickener in expanded fireproof coatings, its impact on storage stability, application thickness performance, film quality, and expanded carbon layer structure, and analyzes its application advantages and performance balance relationship, providing theoretical basis for optimizing the rheological system of fireproof coatings.
The rheological properties and thickening difficulties of expansive water-based fireproof coatings
There are essential differences between expansive water-based fire-resistant coatings and ordinary water-based coatings. Their rheological system has multiple special characteristics such as high filling, multiple electrolytes, complex particles, thick coating construction, and high-temperature foaming into charcoal, which require strict requirements for thickening systems. Firstly, the system contains a large amount of flame-retardant salt powder with high ionic strength and strong electrolyte interference. Traditional ionic thickeners are easily affected by salt precipitation, leading to rheological instability; Secondly, the construction of fireproof coatings mainly involves thick coating and multiple coats, with a single wet film thickness of 300-800 μ m. The system is required to have high anti sagging, high yield value, and moderate leveling properties; Again, during the drying process of coatings, thick film shrinkage stress is high, which can easily lead to cracking, peeling, and cracking, requiring extremely high levels of viscoelastic balance in the system; Finally, thickening agents must not interfere with the synergistic expansion reaction of the flame retardant ternary system (acid source, carbon source, gas source), and must not reduce the expansion height and density of the carbon layer.
Traditional thickening systems generally have shortcomings: cellulose ether thickening systems have too strong rigidity and excessive elasticity, and thick coatings are prone to cracking when dry; HASE alkaline swelling thickening agent is severely affected by high salt systems, and its storage viscosity deteriorates significantly; Bentonite powder can easily cause porosity and decreased density of the paint film, affecting the thermal insulation stability of the carbon layer. In contrast, HEUR belongs to the category of non-ionic associative thickeners, with excellent salt resistance, strong electrolyte stability, and controllable viscoelasticity. It is more suitable for high salt and high filling expansion fireproof coating systems and is currently an ideal rheological additive for thick coating water-based fireproof coatings.
Regulation mechanism of HEUR on rheological properties of expansive fireproof coatings
HEUR constructs a dynamic three-dimensional network in high filling fireproof coating systems by relying on the intermolecular association and particle bridging of hydrophobic end groups, and its rheological response characteristics are significantly different from those of ordinary latex paint systems. In a static low shear state, HEUR hydrophobic groups fully associate to form a uniform, stable, and flexible physical network, significantly improving the yield value and low shear viscosity of the system. It can effectively support a large amount of inorganic flame retardant powders and fillers, suppress high-density particle settling and delamination, and solve the problems of hard precipitation, water separation, and agglomeration in the storage of high solid content fireproof coatings. Under the medium high shear action of brushing, roller coating, and spray coating construction, the hydrophobic binding nodes quickly and reversibly dissociate, the system viscosity rapidly decreases, the flowability of the coating is improved, the thick coating construction is evenly spread, the construction resistance is small, and the brushing feel is smooth.
After the construction is completed, the shear is cancelled, the bonding network is quickly reconstructed and restored, the system viscosity and yield value instantly rise, effectively supporting the thick and wet film structure, greatly improving the anti sagging ability, and solving the industry pain points of easy flow, uneven thickness, and difficulty in hanging fireproof coatings on facades. HEUR endows coatings with flexible viscoelasticity, which is different from the rigid thickening of traditional thickeners. It can effectively release the shrinkage stress during the drying process of thick films, reduce the cracking, cracking, and rolling defects of thick coating films, and significantly improve the integrity of thick film formation.
The influence of HEUR on the storage stability of fireproof coatings
Due to its extremely high powder content and large density difference, expansive fireproof coatings are prone to severe settling, bottom hardening, and upper layer water delamination during storage, which is a difficult point in controlling the stability of coating storage. The uniform flexible binding network constructed by HEUR can comprehensively encapsulate and suspend flame retardant powders and filler particles, allowing particles of different densities and sizes to be uniformly dispersed in the system, significantly reducing the rate of gravity settling. At the same time, HEUR’s non-ionic structure is resistant to salt, electrolyte, and pH fluctuations, and is not affected by ionization interference from flame retardant salts such as ammonium polyphosphate. The system has excellent rheological stability during long-term storage.
Comparative experiments have shown that under the same addition conditions, HEUR thickening fireproof coating has no obvious layering or hard precipitation after being stored at room temperature for 6 months. The viscosity fluctuation during cold and hot cycling storage is small, and the system uniformity is much better than that of cellulose and bentonite systems. At the same time, HEUR will not experience accumulation, compaction, or rough agglomeration like inorganic powders. The can opening state is uniform, and it can be directly constructed without the need for high-strength redispersion, significantly improving the convenience and batch stability of engineering construction.
The Influence of HEUR on the Performance of Thick Coating Construction and the Quality of Film Formation
The construction of expansion type fireproof coating engineering takes multiple thick coatings as the core process, with a large single wet film thickness and a long film formation cycle. The paint film is prone to defects such as sagging, cracking, pinholes, orange peel, and uneven thickness. HEUR’s unique shear thinning and rapid recovery rheological properties perfectly meet the requirements of thick coating construction. High shear reduction ensures good flowability, uniform spreading, and high smoothness of the paint film during coating application; Shear recovery quickly enhances the wet film support force, effectively suppresses the gravity flow of thick wet film, significantly increases the maximum coating thickness for a single construction, reduces the number of construction passes, and improves the efficiency of engineering coating.
At the same time, the main chain of HEUR molecules is a flexible polyether structure, which can moderately enhance the flexibility of the paint film after film formation, alleviate the drying shrinkage stress of thick paint films, and effectively solve the common problems of rapid surface drying, uneven internal water loss, and drying cracking in thick fireproof coatings. Compared to traditional thickening systems, the HEUR system has a higher overall density, smoother surface, and no obvious pore defects in the paint film. It can effectively block water vapor penetration, improve coating adhesion and long-term durability stability, and avoid problems such as peeling, peeling, and powdering of the paint film in the later stage.
The influence and balance relationship of HEUR on the properties of expansion foaming and carbon layer
Unlike ordinary decorative coatings, the core properties of fireproof coatings are expansion ratio, carbon layer density, and thermal insulation stability. The addition of rheological agents inevitably has positive and negative effects on the foaming system, and there is a clear performance balance relationship, which is also a key control point in the formulation design of fireproof coatings.
Moderate HEUR can optimize the micro uniformity of the paint film, making the ternary flame retardant system of acid source, carbon source, and gas source more evenly dispersed. When heated at high temperatures, each component reacts synchronously and foams uniformly, forming a structurally regular, dense, and continuous expanded carbon layer. The carbon layer has no large pores, cracks, or collapses, and its thermal insulation performance is stable. But when the amount of HEUR added is too high, the system’s binding network becomes too strong, the paint film viscosity is too high, and the high-temperature melt flowability deteriorates, which will inhibit the coating’s high-temperature free expansion, resulting in a decrease in expansion ratio, a decrease in carbon layer thickness, insufficient foaming, and directly weaken the fire insulation effect. At the same time, excessive enrichment of organic polymers can cause the residual carbon in the carbon layer to become soft, weak, prone to pulverization and detachment, and unable to form a stable protective carbon layer at high temperatures, seriously reducing the fire resistance limit.
In addition, excessive content of HEUR hydrophilic polyether segments can slightly improve the hydrophilicity of the paint film, while excessive addition can lead to a slight decrease in the water resistance of the coating. Long term service in humid and hot environments is prone to water absorption softening and adhesion degradation problems. Therefore, there is a strict optimal addition window for HEUR in expansion fireproof coatings, which needs to balance rheological construction performance and fireproof foaming performance.
Existing problems in the application of expansion fireproof coating HEUR
Although HEUR is compatible with thick coated fireproof coating systems, there are still significant shortcomings in practical applications. Firstly, the low shear yield value of pure HEUR is limited, and the anti settling ability of a single HEUR with ultra-high solid content and ultra thick coating system is still insufficient. There is a slight soft settling phenomenon during long-term storage; Secondly, HEUR is still sensitive to surface active agents and excessive wetting agents in the system, and high additive systems are prone to rheological fluctuations; Thirdly, excessive addition suppresses foaming expansion, and there is a natural balance between rheological properties and fire resistance, resulting in a narrow formula tolerance rate; Fourthly, ordinary HEUR has limited high-temperature resistance, and the high-temperature carbonization process will prematurely decompose, which has a slight negative impact on the density of the carbon layer.
Optimize application strategies and complex system solutions
In response to the shortcomings of HEUR in the application of expansive water-based fire-resistant coatings, the industry generally adopts the technical ideas of precise quantity control, synergistic compounding, and system optimization to achieve a balance and unity of construction performance, storage performance, and fire resistance.
Firstly, the HEUR addition range is strictly controlled, and low addition amounts and high-efficiency associative industrial HEUR are used to minimize interference with the foaming system while ensuring rheological stability, while also meeting the requirements of thick coating anti sagging and high expansion ratio. Secondly, a composite system of HEUR and trace inorganic rheological additives is constructed, utilizing a small amount of modified bentonite and magnesium aluminum silicate to provide stable yield values, enhance anti settlement and anti delamination capabilities, and use HEUR to ensure construction leveling and thick coating without cracking, achieving complementary rheological properties. Again, optimize the formulation additive system, strictly control the excessive addition of wetting and dispersing agents, reduce the interference of small molecule surfactants on the HEUR binding network, and stabilize the rheological structure of the system. Finally, priority should be given to using high temperature resistant and low hydrophilic modified HEUR to enhance the water resistance and high temperature thermal stability of the paint film, and reduce the damage to the carbon layer structure caused by the thermal decomposition of the additives themselves.
Summary
HEUR polyurethane associative thickener, with its strong salt resistance, significant shear thinning, excellent viscoelasticity, and good compatibility for thick coating, effectively solves the problems of storage settling, thick coating sagging, drying cracking, and poor film quality of traditional thickeners in expansive water-based fireproof coatings. It can significantly improve the storage stability and thick coating construction performance of fireproof coatings. However, the amount of HEUR added has a significant balancing effect on the expansion and foaming of the coating, the structure of the carbon layer, and the fire resistance limit. Excessive addition can inhibit the expansion of the coating, reduce the quality of the carbon layer, and lower the thermal insulation performance. By precise selection, strict control of addition amount, and construction of organic-inorganic composite rheological system, the rheological construction performance, storage stability, and fire insulation performance of fireproof coatings can be effectively balanced, providing important support for the formulation development and engineering application of high-performance, high stability, thick coating water-based expansion fireproof coatings.
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Post time: Sep-07-2026