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Role of Activated Molecular Sieve Powder in Epoxy Zinc-Rich Coating Systems
In epoxy zinc-rich coating systems, activated molecular sieve powder is primarily used as an efficient dehydrating agent to adsorb trace moisture within the system. This prevents moisture from reacting with active zinc powder to generate hydrogen gas, which can cause excessive internal can pressure, while also avoiding bubble formation and extending coating storage stability. Below are the detailed usage methods, addition levels, addition temperatures, and process key points:
## I. Mechanism of Action and Grade Selection
### 1.1 Core Functions
- **Dehydration and Gas Prevention**: Adsorbs trace moisture from raw materials (solvents, pigments, additives), inhibiting the reaction between water and zinc powder to generate hydrogen gas, thereby preventing excessive can pressure or even leakage.
- **Defoaming and Anti-foaming**: Reduces CO₂ bubble generation caused by trace moisture reacting with isocyanates or epoxy systems, minimizing risks of pinholes and blistering in the coating film.
- **Extended Storage Stability**: Reduces system moisture content, slowing down prepolymer hydrolysis or degradation during storage.
### 1.2 Recommended Grades
| Grade | Effective Pore Diameter | Characteristics | Application Scenario |
| **3A** | 0.3 nm | Precisely adsorbs water molecules (kinetic diameter ~0.26 nm), **does not adsorb organic molecules**, offering the best selectivity | **First choice**, especially suitable for systems containing isocyanates or highly active zinc powder |
| **4A** | 0.4 nm | General-purpose, lower cost, slightly higher water adsorption capacity | Conventional epoxy zinc-rich coatings |
| 5A/13X | Larger pore diameter | Can adsorb CO₂, H₂S, and other gases | Special purification requirements |
> Waterborne epoxy zinc-rich coating patents explicitly specify the use of **3A-type and/or 4A-type** activated molecular sieve powder.
## II. Addition Level
### 2.1 Reference Dosage
- **Patent formulation reference**: In waterborne epoxy zinc-rich coating Component A, the addition level is **0.3–1 parts by weight** (based on the total weight of Component A).
- **General industrial recommendation**: Typically **0.5%–3%** of the total coating system weight, specifically determined according to the original moisture content of the system.
- **Theoretical calculation**: The static water adsorption capacity of activated molecular sieve powder can reach **≥24%** (RH50%, 25°C), meaning 1 g of activated powder can theoretically adsorb approximately 0.24 g of water.
### 2.2 Calculation Principle
Molecular sieve addition level ≈ Total system moisture content ÷ 20% (considering adsorption efficiency margin)
> In practice, it is recommended to determine the optimal level through small-scale trials, using the criteria of no gas swelling and no abnormal viscosity increase after coating storage.
III. Addition Temperature and Process
### 3.1 Temperature Control
- **Dispersion temperature**: **≤40°C** (optimal control at ambient temperature to 35°C).
- Patent processes explicitly specify conducting operations in a reactor with circulating condensate water, maintaining slurry temperature **below 40°C**.
- Excessive temperature (>50°C) may accelerate the epoxy-amine reaction, shortening the pot life.
- **Heat resistance limit**: Activated molecular sieve powder itself can withstand temperatures **below 500°C**, but the coating system should not be operated at high temperatures.
### 3.2 Recommended Process Flow (Two-Component Waterborne Epoxy Zinc-Rich Coating)
**Component A Preparation (containing activated molecular sieve powder):**
| Step | Operation | Speed | Time | Temperature |
| 1 | Charge waterborne epoxy resin, dispersant, wetting agent, and partial additives into the reactor | 200–400 rpm | 10–20 min | Ambient |
| 2 | Add deionized water (if required), mix uniformly | 400–600 rpm | 10–20 min | Ambient |
| 3 | **Add activated molecular sieve powder**, anti-settling agent, defoamer | **1000–2000 rpm** | **15–30 min** | **<40°C** |
| 4 | Add hollow glass microspheres, zinc powder, anti-corrosion pigments | 1000–2000 rpm | 15–30 min | <40°C |
| 5 | Grind to fineness **≤60 μm** (some requirements ≤50 μm) | Bead mill | Equipment-dependent | <40°C |
| 6 | Add remaining additives (thickener, leveling agent, etc.), adjust viscosity | 400–600 rpm | 15–20 min | Ambient |
**Component B Preparation:**
- Mix waterborne amine curing agent, flash rust inhibitor, and water, stir at 1000–1500 rpm for 15–30 min.
**Maturation and Application:**
- Component A:B mass ratio is typically **(5–15):1**.
- After mixing, maturation temperature **20–40°C**, maturation duration **0.5–4 h**.
### 3.3 Solvent-Based Epoxy Zinc-Rich Coating Process Reference
For solvent-based epoxy zinc-rich coatings, activated molecular sieve powder is typically added during the high-speed dispersion stage together with zinc powder, anti-corrosion pigments, and other powders:
- First charge epoxy resin, solvent, and dispersant, stir at medium speed.
- Add activated molecular sieve powder, zinc powder, and fillers, pre-disperse at **600–800 rpm** for 10–15 min.
- Raise temperature to **45–60°C** for high-speed dispersion to fineness ≤35 μm.
- Adjust paint, filter, and package.
## IV. Key Precautions
| Item | Requirement |
| **Package Opening** | Activated molecular sieve powder is a desiccant; **do not open the package until the moment of use**, as exposure to air will rapidly cause moisture adsorption and deactivation. |
| **Addition Timing** | Should be added **early during the powder dispersion stage** to ensure adequate dispersion time; avoid late addition which may cause uneven distribution. |
| **Dispersion Equipment** | High-speed disperser or reactor with circulating cooling is recommended to prevent excessive temperature rise. |
| **Storage Conditions** | Unused activated powder should be sealed and stored in a dry environment to prevent moisture absorption. |
| **Viscosity Impact** | Activated molecular sieve powder **does not have thickening properties**; its effect on system viscosity is only the general impact of an inorganic filler. |
| **Water Release Concern** | **Will not release adsorbed water**. In the coating system, it forms a uniform substance with the resin and lacks the high-temperature/low-pressure conditions required for desorption; it is an irreversible physical adsorption. |
| **Chemical Inertness** | Pure physical adsorption mechanism; does not participate in chemical reactions and does not affect coating curing performance or film properties. |
## V. Typical Technical Specifications Reference
| Specification | 3A Type | 4A Type | Test Conditions |
| Static Water Adsorption | ≥22.5% | ≥26% | RH50%, 25°C |
| Packaged Moisture Content | ≤2.0% | ≤2.0% | 550°C, 2 h |
| Sieve Residue (325 mesh) | ≤1.0% | ≤1.0% | — |
| pH Value (1% solution) | ≤11 | ≤11 | — |
| D50 Particle Size | 3–5 μm | 3–5 μm | Laser particle size analyzer |
| Bulk Density | ≥0.43 g/ml | ≥0.43 g/ml | — |
## VI. Summary Recommendations
1. **Prioritize 3A-type** activated molecular sieve powder, as it offers high selectivity for water molecules without adsorbing organic components, making it most suitable for epoxy zinc-rich systems.
2. **Start trials at 1% addition level** and adjust based on actual dehydration effectiveness; generally do not exceed 3%.
3. **Strictly control dispersion temperature <40°C**; use high-speed dispersion (1000–2000 rpm) for 15–30 min to ensure uniform distribution.
4. **Pay attention to moisture protection**: Use immediately after opening to prevent pre-adsorption of moisture by the activated powder.
5. For high zinc content coatings (>70%), it is recommended to appropriately increase the molecular sieve dosage, as zinc powder has a large specific surface area and is more sensitive to moisture.
Dalian Chuangge Technology Co., Ltd
Richard Han
Doris Zhou
Headquater: 9th Floor, Century Classic Building, No.10, Xinghai Square, Shahekou District, Dalian City, Liaoning Province
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