Introduction: The Dual Mission of Fire Resistance and Corrosion Protection
High-rise building exterior steel structures face the dual requirements of fire protection + weather-resistant anti-corrosion. Intumescent fire-resistant coating (dry film 1-3mm) serves as the intermediate layer, with its rough surface, uneven thickness (deviation of ±0.5mm may occur at irregular steel structure parts), and significant difference in thermal expansion coefficient (about 50×10⁻⁶/°C) from fluorocarbon topcoat (about 30×10⁻⁶/°C) — these factors together cause the topcoat on the fire-resistant coating surface to be highly prone to tensile cracks (Cracking) — caused by the expansion/contraction difference between the fire-resistant coating/topcoat leading to the topcoat being “torn apart”.
I. Design of Fireproof and Anti-corrosion Composite Coating System
| Coating Layer | Product Type | Dry Film Thickness (μm) | Function |
|---|---|---|---|
| Primer | Epoxy Zinc-Rich Primer | 60-80 | Cathodic protection + anti-corrosion |
| Intermediate Coat | Epoxy Micaceous Iron Oxide Intermediate Paint | 80-120 | Barrier anti-corrosion + transition layer |
| Fireproof Layer | Intumescent Fireproof Coating (solvent-based) | 1000-3000 | Fire insulation and intumescence |
| Sealer (optional) | Epoxy Sealer | 30-50 | Seal rough surface of fireproof layer (recommended) |
| Topcoat | FEVE Fluorocarbon Topcoat / Aliphatic PU Topcoat | 50-80 | Weather resistance and decoration |
II. Requirements for Fireproof Coating Thickness of Structural Steel with Different Fire Resistance Ratings
| Fire resistance rating (h) | Intumescent thickness (mm) | Non-intumescent thickness (mm) | Steel section factor (m⁻¹) |
|---|---|---|---|
| 0.5 | 0.5-1.0 | 5-10 | >200 |
| 1.0 | 1.0-1.5 | 10-15 | 150-200 |
| 1.5 | 1.5-2.0 | 15-25 | 100-150 |
| 2.0 | 2.0-3.0 | 20-30 | 80-100 |
| 3.0 | 3.0-4.0 | 30-45 | <80 |
Key Control Point: The seal coat (epoxy sealer) between the fireproofing coating and the topcoat is not mandatory but is strongly recommended. The seal coat has three major functions: (1) filling the 1-3mm surface unevenness of the fireproofing coating (self-leveling and self-filling); (2) providing an elastic buffer layer to compensate for the thermal expansion difference between the fireproofing and topcoat; (3) enhancing adhesion between the fireproofing coating and the topcoat—the fireproofing coating contains a large amount of APP/PER/MEL fillers, has a loose and porous surface, and direct topcoat spraying results in poor adhesion.
Technical deepening: systematic optimization methods for process parameters (DOE experimental design)
Coating production process optimization should not rely on the “trial-and-error method” but should adopt the scientific method of DOE experimental design. Taking the dispersion process as an example—factors affecting quality (linear velocity/time/filling rate/temperature), 4 factors each at 3 levels—full factorial requires 81 experiments—DOE uses orthogonal experiment L9 (9 times) or response surface methodology (27 times) to greatly reduce the number of experiments—while obtaining the main effects and interactions of each factor. For example, it is found that “the interaction of linear velocity × time is significant”—high linear velocity + short time and low linear velocity + long time can achieve the same dispersion effect—but the former saves energy by >20%.
In DOE analysis, interpretation of the P-value — P95% confidence). The final output of DOE is a set of prediction models (polynomial regression equations) — input line speed/time/temperature → predict fineness/viscosity/gloss — providing formulation engineers with a ”digital formulation optimization” tool.
Industry practice: from “master craftsman’s feel” to “parameter standardization”
The common challenge in the coatings industry—when experienced veteran workers retire, their “feel” (mixing resistance / fineness gauge scraping / visual inspection of wet film gloss) is taken away—new employees cannot replicate it. Transform the “feel” into quantifiable standard parameters (1) mixing resistance → viscometer reading; (2) fineness gauge scraping → fineness gauge reading (μm); (3) wet film gloss → gloss meter (GU value). The “standard parameter card” for each process is posted next to the equipment—new employees operate according to the “card” rather than “by feel”. “Parameter standardization” is a key step for coating factories to move from “workshop” to “factory”.
FAQ
Q1: Must fireproof coatings be solvent-based rather than water-based?Almost all outdoor steel structure fireproof coatings use solvent-based formulations—because water-based fireproof coatings have a high water absorption rate (>5%), and outdoor rainwater immersion will cause APP hydrolysis failure (a sharp drop in expansion ratio) and coating softening and detachment. Water-based fireproof coatings are limited to indoor (relative humidity <80%) applications.
Q2: How to prevent surface paint cracking caused by uneven thickness of fireproof coating?(1) Apply the fireproof coating in 3-4 passes (each pass ≤1mm), controlling the uniformity of each pass; (2) Lightly sand (120# sandpaper) and level the surface of the fireproof coating; (3) Apply an epoxy sealing layer (30-50μm) to fill and level the surface; (4) Add a toughening agent (flexible polyester resin 5%-10%) to the topcoat formulation to increase the elongation at break to ≥5%.
Q3: How to ensure the adhesion between fireproof coating and topcoat?The pull-off test (ISO 4624) requires ≥2MPa and the failure mode should be cohesive failure within the coating (not interfacial failure). Common causes of insufficient adhesion: (1) The surface of the fireproof coating is loose and porous (weak mechanical anchoring); (2) The loose particles of APP/MEL/PER in the fireproof coating form a weak interfacial layer—this is where the necessity of a sealer coat lies.
Q4: Does intumescent fireproof coating need to be completely removed and repainted after a fire? Yes. Intumescent fireproof coating is a “single-use” functional coating—it expands to form a charred layer during a fire (fulfilling its fireproofing mission), and after the fire the charred layer is completely destroyed (loose, cracked, loss of adhesion), so it must be entirely removed by sandblasting down to the primer layer → then the fireproof coating and topcoat must be reapplied.
Q5: Expected service life of fireproofing + fluorocarbon system?The complete system of epoxy primer + epoxy intermediate paint + fireproof coating + fluorocarbon topcoat is expected to serve 15-20 years in C3-C4 environments (fireproof performance should be tested every 5-8 years to check whether the expansion ratio has degraded by >30% to confirm whether refurbishment is needed). Compared with the pure anti-corrosion coating system without fireproofing requirements (25 years), the service life is shortened — because the presence of the fireproof coating introduces a weak layer in the system (compatibility between the fireproof layer and the topcoat).
Q6: How to comply with VOC emissions of solvent-based fireproof coatings?Solvent-based fireproof coatings usually have a VOC content of 300-500 g/L, far exceeding the limits for industrial protective coatings in GB 30981-2020. Countermeasures: (1) Use RTO/activated carbon adsorption units to treat VOC exhaust gas during construction; (2) High-solid fireproof coatings (solid content >80%) can reduce VOC to <200 g/L; (3) Use water-based fireproof coatings for indoor scenarios to replace solvent-based ones.
Q7: When can topcoat be applied after fireproof coating is painted?It depends on the degree of thorough drying of the fireproof coating. Under standard conditions (23°C/50% RH): solvent-based fireproof coating can be topcoated after 24-48h of thorough drying. However, thick-coat type (>2mm) may require more than 72h. Judgment criterion: the surface hardness of the fireproof coating layer reaches a level that can withstand the impact of topcoat spraying (no obvious mark left when pressed with a fingernail).
Q8: Whole-life cost analysis (LCCA) of the fireproofing/anti-corrosion system?Based on a 20-year service life: the initial construction cost of the fireproofing/anti-corrosion system is about 200-300 RMB/m² (including surface treatment + primer + intermediate coat + fireproofing + sealer + topcoat), and the average annual maintenance cost is about 10-15 RMB/m² (including periodic inspection + local repair). The initial cost of a pure anti-corrosion system (without fireproofing) is about 100-150 RMB/m². The fireproofing function brings about twice the initial investment — this is a mandatory safety investment for steel structure buildings, not an optional item.
Q9: Will applying fluorocarbon topcoat over a fireproof coating reduce the intumescent performance of the fireproof coating?The combination of topcoat + sealer will cause a 5-10 second delay in the initial expansion rate of the fireproof coating (heat needs to conduct through the topcoat/sealer to the fireproof layer). However, the final expansion ratio and char layer thickness will not be significantly reduced. The total thickness of sealer + topcoat should be controlled at ≤120μm — an overly thick (>200μm) topcoat/sealer system may significantly delay the thermal response of the fireproof coating.
Q10: In which scenarios is intumescent fireproof coating not suitable?(1) Steel structures长期处于水中 (such as ship locks, water gates) — APP undergoes irreversible hydrolysis; (2) External surfaces of equipment operating continuously at high temperatures >80°C — APP decomposes prematurely; (3) Areas with frequent mechanical impact — the mechanical strength of intumescent coating is weaker than that of ordinary coating.
FAQ: In-Depth Technical Q&A Supplement
Q11: How do the differences in domestic and international standards for this technology affect product export?Domestic standards (GB) differ from ISO/ASTM standards in test methods and acceptance criteria. For example, salt spray testing—GB/T 1771 (equivalent to ISO 7253) has test conditions basically consistent with ASTM B117—but the rating systems (ISO 4628 vs ASTM D610/D714) differ—when providing test reports for exported products, the corresponding international standards must be indicated simultaneously, otherwise overseas customers cannot conduct a comparative evaluation. It is recommended to list both GB and ISO/ASTM dual-standard indicators in the TDS (Technical Data Sheet) of exported products—to enhance the trust of international customers.
Q12: How to verify the long-term service performance of this technology in actual engineering?Laboratory accelerated testing (salt spray/QUV/cyclic corrosion) provides comparative data—but cannot fully replace actual outdoor exposure testing. Recommendations—(1) Set up outdoor exposure racks at both the factory location and typical customer locations (e.g., coastal C5-M/industrial C4)—conduct annual inspections of coating appearance/adhesion/film thickness changes—establish a company-owned outdoor service database; (2) Collaborate with universities/research institutes—combine enterprise data with academic research—enhance data credibility.
Q13: What should SMEs pay attention to when purchasing related raw materials/equipment?(1) The batch stability of suppliers is more important than unit price—it is recommended to require suppliers to provide COA data for >10 batches—and evaluate batch variation (CpK); (2) For equipment procurement, visit peers who have used the equipment for >2 years to understand the long-term reliability and after-sales service quality of the equipment—rather than relying only on the demonstration data from equipment suppliers; (3) For critical raw materials (resin/curing agent)—maintain at least 2 qualified suppliers to guard against single-source supply risk.
Q14: What is the current state and trend of digital transformation in this field?The digital transformation of the coatings industry is evolving from “point-based applications” (automation of individual equipment/processes) to ”system integration” (full-chain ERP+MES+PMS). Currently, for small and medium-sized coatings factories, the digitalization with the ”highest ROI investment” is automatic batching systems + digitalization of quality control data—with a payback period of 1–3 years—which is the prioritized recommended direction. Future trend—AI + sensors enabling real-time optimization of process parameters—further reducing quality fluctuations between batches.
Q15: How can a newly entered coating engineer quickly master this technology?(1)Combine theory and practiceDo not only read literature without touching actual production—nor rely solely on experience without studying theory;(2)Establish a “failure case archive”Every customer complaint/production anomaly/coating failure—record the root cause and resolution process—this is the most effective learning material;(3)Learn from suppliersTechnical personnel from resin/additive/pigment suppliers are carriers of “tacit knowledge” in this field—communicate more with them about solutions to specific problems.
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Summary
The composite coating system of intumescent fireproof coating for outdoor steel structures plus fluorocarbon topcoat is the mainstream technical solution for high-rise buildings, stadiums, and airport steel structures. Adding an epoxy sealer layer (30–50 μm) between the fireproof layer and the topcoat is key to ensuring interlayer adhesion and compensating for thermal expansion differences. Kexin New Materials provides customers with a complete product line and technical support for fireproof/anti-corrosion composite coating systems.