Manufacturing robust components through casting requires specialized coatings that withstand extreme thermal and mechanical stresses, particularly for large casting production. Waterborne spray coatings offer significant advantages over solvent-based alternatives, including reduced environmental impact, lower energy consumption during drying, and enhanced operator safety. For large casting applications, where manual brushing becomes impractical and efficiency is paramount, spray application enables uniform coverage on complex geometries and massive surfaces. This study systematically examines the formulation parameters critical for high-performance corundum-based waterborne spray coatings specifically engineered for large casting operations.
Experimental Methodology
White corundum (≥99% Al2O3) served as the primary refractory aggregate. Attapulgite clay acted as the suspension agent, while polyvinyl alcohol (PVA) and carboxymethyl cellulose sodium (CMC) functioned as binders. A modified silicone polymer (EFKA 3288) enhanced spray characteristics, and zinc oxide (ZnO)/cerium oxide (CeO2) mixtures were evaluated as sintering aids. Key instruments included a high-speed disperser, rotational viscometer, coating abrasion resistance tester, and sintering point analyzer.
Coatings were prepared through sequential dispersion. Attapulgite underwent pre-dispersion in water (1:4 ratio). CMC and PVA were separately dissolved in water at 60°C and 80°C, respectively. The main dispersion tank received water, preservative (sodium pentachlorophenate), defoamer, and pre-dispersed attapulgite slurry, mixed at 1200 rpm for 20 minutes. CMC and PVA solutions were added sequentially, followed by other additives. White corundum was incorporated gradually, and the mixture dispersed further. Performance was assessed via:
- Shear Thinning Index (STI): Ratio of viscosity at 6 rpm (η6) to viscosity at 60 rpm (η60) measured using a rotational viscometer: $$ STI = \frac{\eta_6}{\eta_{60}} $$ Higher STI indicates better sprayability.
- Abrasion Resistance: Mass loss (g) after 4 revolutions using a standardized abrasion tester (JB/T 9226-2008).
- Sintering Point: Temperature at which refractory particles began fusing, determined using a sintering point analyzer.
- Conditional Viscosity: Efflux time (s) measured using a 4mm or 6mm orifice cup.
- Suspension Stability: Percentage settled solids after 24 hours.
Results and Discussion
Suspension Agent Optimization
Attapulgite clay concentration critically influences rheology and stability. As Table 1 shows, suspension stability improves with increasing attapulgite, but excessive amounts elevate viscosity detrimentally. Optimal sprayability and stability for large casting coatings balance these factors.
| Attapulgite (%) | Suspension (24h, %) | Shear Thinning Index (STI) | Conditional Viscosity (6mm, s) |
|---|---|---|---|
| 2.0 | 93 | 6.5 | 9.0 |
| 2.5 | 96 | 7.2 | 9.5 |
| 3.0 | 97 | 7.5 | 11.0 |
| 3.5 | 98 | 7.1 | 13.0 |
Equation 1 models the viscosity-suspension relationship: $$ \eta = k \cdot C_a^{1.5} $$ where η is viscosity, Ca is attapulgite concentration, and k is a constant. The optimal range of 2.5–3.0% provides ≥96% suspension and STI >7.0, crucial for maintaining homogeneity during spraying of voluminous large casting molds.
Binder System Selection
Binder concentration governs coating strength, gas evolution, and application behavior. PVA primarily enhances mechanical strength, while CMC modifies viscosity. Elevated binder levels increase abrasion resistance but also raise gas evolution, risking casting defects like porosity – a critical concern in thick-section large casting.
| PVA (%) | CMC (%) | STI | Abrasion Loss (4r, g) | Conditional Viscosity (4mm, s) | Gas Evolution (mL/g) |
|---|---|---|---|---|---|
| 0.5 | 0.2 | 6.8 | 0.41 | 7.0 | 15.2 |
| 1.0 | 0.2 | 7.2 | 0.31 | 9.5 | 17.6 |
| 0.5 | 0.5 | 7.0 | 0.30 | 11.0 | 18.0 |
| 1.0 | 0.5 | 7.3 | 0.26 | 13.0 | 20.8 |
The formulation with 1.0% PVA and 0.2% CMC delivered optimal balance: STI=7.2, abrasion loss=0.31g, and moderate gas evolution (17.6 mL/g), essential for minimizing defects in high-value large casting components.
Spray Modifier Effectiveness
High-solids coatings (>90°Bé) used in large casting spraying require exceptional shear-thinning to prevent nozzle clogging. EFKA 3288, an organosilicon modifier, significantly enhances STI but reduces solids content. Table 3 demonstrates this trade-off.
| EFKA 3288 (%) | STI | Solids Content (%) |
|---|---|---|
| 0.0 | 4.5 | 74.6 |
| 0.2 | 5.8 | 74.1 |
| 0.4 | 6.6 | 73.3 |
| 0.6 | 7.0 | 72.0 |
| 0.8 | 7.2 | 68.4 |
The relationship between STI and modifier concentration follows: $$ STI = STI_0 + \alpha \cdot C_m $$ where STI0 is the base index, α is an enhancement coefficient, and Cm is modifier concentration. Concentrations of 0.4–0.6% achieved STI≥6.6 while maintaining solids content >72%, ensuring efficient deposition and drying for large casting molds.
Dispersion Time Optimization
Adequate dispersion ensures complete particle wetting and stabilizes rheological properties. Prolonged dispersion slightly improved STI and suspension but offered diminishing returns beyond 30 minutes (Table 4), impacting production efficiency for large casting operations requiring bulk coating volumes.
| Dispersion Time (min) | STI | Conditional Viscosity (6mm, s) | Suspension (24h, %) |
|---|---|---|---|
| 20 | 6.6 | 10.0 | 96 |
| 30 | 7.0 | 9.5 | 97 |
| 60 | 7.2 | 9.5 | 97 |
Sintering Aid Development for Corundum
Pure white corundum’s high sintering point (1882°C) exceeds typical steel large casting pouring temperatures (1630–1650°C), leading to poor sintering, metal penetration (mechanical burn-on), and difficult shakeout. Single-component sintering aids (ZnO or CeO2) provided insufficient reduction (Table 5). A synergistic ZnO/CeO2 mixture dramatically lowered the sintering temperature.
| Sinter Aid Composition | Sintering Point (°C) | Reduction vs. Baseline (°C) |
|---|---|---|
| None | 1882 | 0 |
| ZnO | 1750 | 132 |
| CeO2 | 1782 | 100 |
| ZnO:CeO2 (1:1) | 1725 | 157 |
| ZnO:CeO2 (2:1) | 1656 | 226 |
| ZnO:CeO2 (4:1) | 1625 | 257 |
| ZnO:CeO2 (6:1) | 1637 | 245 |
The 4:1 ZnO/CeO2 ratio achieved the lowest sintering point (1625°C), 25°C below the minimum pouring temperature. This promotes formation of a protective vitreous layer during large casting, described by the liquid-phase sintering model: $$ \frac{d\rho}{dt} = \frac{K \gamma \Omega D}{kT r^3} $$ where dρ/dt is densification rate, K is a constant, γ is surface energy, Ω is atomic volume, D is diffusivity, k is Boltzmann’s constant, T is temperature, and r is particle radius. The additive forms a low-melting eutectic phase, facilitating particle rearrangement and pore elimination.
Conclusion
This study established an optimized formulation and process for high-performance corundum-based waterborne spray coatings tailored for large casting applications. Key parameters include: 2.5–3.0% attapulgite suspension agent, 1.0% PVA and 0.2% CMC binders, 0.4–0.6% EFKA 3288 spray modifier, and a 30-minute dispersion time. Crucially, a 4:1 ZnO/CeO2 sintering aid (0.8% total) reduced the sintering point to 1625°C, enabling effective vitrification during steel pouring. The resulting coating exhibits high suspension stability (≥96%), excellent shear-thinning behavior (STI≥7.0), adequate dry strength (abrasion loss ≤0.31g), controlled gas evolution, and superior anti-penetration properties. This formulation meets the stringent demands of industrial large casting production, offering significant advantages in application efficiency, casting quality, and environmental footprint compared to traditional systems.